NAM favors freezing rain

New short term model guidance is favoring slightly stronger cold air damming at the surface. This translates to the possibility of a more prolonged period of freezing rain, perhaps lasting through 6am. Model QPF suggests up to a tenth inch of accretion is possible. By 6am, surface temperatures should be above freezing, with plain rain.

(Added at 930pm) As for delay potential, it's another somewhat tricky forecast, because just a half of a degree in surface temperature makes a huge difference. However, I do not think road conditions tomorrow morning will be worse or even comparable for that matter to this past Monday morning.

Icy Tuesday Night

As suggested, strong lift combined with abundant moisture in the snow growth zone is supporting large flakes that are accumulating quickly. At my house at 1000ft, already amounts are approaching 2". Snow, heavy at times, will continue through noon, with totals of 4-5" below 1000ft to 5-8" above 1000ft.

The next storm system of interest is currently developing in the southwest, and will lift northeast toward the Great Lakes region tomorrow and tomorrow night. We will have high pressure to our north again, but it will be retreating as warm advection moves into the region. Precipitation will begin overspreading the area after 9pm, in the form of snow. Around an inch of accumulation is possible before mixing with and changing to sleet and freezing rain after midnight. Freezing rain, sleet, and rain will continue through 9am Wednesday until warm aid advection completely scours out all cold air. Precipitation rates will be fairly light, but that is all the more dangerous when it comes to freezing rain. Ice accretion around a tenth of an inch is possible by the time of the morning commute.

Upstream radar looks good

Definitely a nice slug of moisture coming east. There are some differences between the GFS and NAM placement of max QPF. However, both deliver .5"+ to SNH. Precipitation will begin moving in after midnight tonight. There will be a fairly large elevated warm layer that will support liquid precipitation through the mid levels through 3am to 5am. Surface temperatures are between 35 and 40 and will probably be hovering around freezing by 3am. This will be inadequate to support freezing rain, so rain, sleet, and snow will be the primary precipitation types for this storm. The warm layer will be shinking enough as precipitation begins to avoid much longer than an hour or two of rain in the beginning, so should be mainly sleet/snow through 6am, with accumulations around an inch.

Still looks like good snow growth for the morning hours supporting large flakes. This should overcome earlier melting on pavement, and roads will likely begin to get snow covered after 9am. GFS and NAM agree on a fairly large amount of available moisture, however the GFS is further south with the placement of max QPF. This is the difference between 3-5 and 5-9 in Keene. SREF probabilities show good support for 4+" across SNH. I still like my original thoughts of 4-5 lower elevations, 5-7 higher elevations.

--- Snowday Outlook ---
Definitely a tricky forecast. Roads will be slippery by 6am tomorrow, but shouldnt be snow covered yet. By noon, however, travel conditions will be likely be much more hazardous especially on untreated roads. Still, all efforts will probably be made to not have another snowday.

Moderate snow event Monday

Forecast model guidance continues to advertise a large slug of moisture entering the region tomorrow morning in association with a warm advection pattern in advance of low pressure to our southwest. High pressure will slide south across Quebec tonight, reinforcing cold air at the surface. Precipitation will move in after 3am tonight in the form of snow/sleet north of Rt 2, sleet north of I90, and generally rain to the south of I90, with some high elevation sleet moving in by 6am. Low pressure will track east from the Great Lakes into southwest NY, with a triple point low developing over New Jersey. The slope of surface pressure from northwest to southeast will result in the colder temperature profiles to actually be toward the northeast versus northwest which is more common. As secondary low takes over, the snow/sleet/rain lines will sag to the south through Monday morning.

This will be a fast moving system, and most precipitation will be out of western New England by 2pm, with some snow lingering in eastern New England in association with an inverted trough. H85 and H7 low track support max accumulations to align from central VT into central and southern NH.

For Keene: Snow/sleet should start after 3am, with an inch by 6am. Precipitation should shift to all snow after 6am, and could fall at a decent clip through 11am. Snow growth looks good for the area, which should support large flakes that accumulate quickly through the morning. Snow should wind down after noon. As with most spring storms, elevation makes a difference. Total accumulation below 1000ft will be around 4 to 5 inches, with 5 to 7 above 1000ft.

--- Snowday Outlook --- Added at 12:40pm
Timing is a little late for high confidence. Bulk of the snow will be falling between 8am and 11am. This timing is alright for a cancellation, but nothing else. Keep that in mind: this looks like an all or nothing deal. Amounts are good enough IF timing were to be a little earlier. In addition, the fact that it is later in the season with 7 snowdays on the board already means that subsequent cancellations are progressively harder to come by.

Round of snow/sleet Monday

Low pressure currently over Kansas will move into northern Illinois Sunday afternoon and through northwest PA Sunday night. An associated warm front will lift toward the region, with increasing warm air advection and widespread overrunning precipitation by Monday morning. At this point, it looks like thermal profiles support the bulk of the precipitation as snow and sleet north of Rt 2. Model guidance is showing high QPF for much of the region, on the order of .6" to 1" through Monday afternoon. I haven't been able to take a very close look at everything yet, but my initial thoughts are 3" to 6" of snow for Keene.

--- Snowday outlook ---
At this point all I can say is that timing and amounts look decent for delay/cancellation potential on Monday. I will have more information tomorrow morning.

Lighter still

Continues to look like the main region of precipitation will stick to northern New England. Model guidance has backed off on QPF, with consensus for under a tenth inch, and even struggling to reach measurable accumulation. Generally, looking at a coating of snow with some sleet and freezing rain possible in the morning. Timeframe will be centered around the morning commute, so regardless of trace amounts, there still could be impact on travel.

Light wintry precip Friday

Low pressure moving through the northern plains tomorrow will lift a warm front toward the region Thursday night. Warm air advection and moisture advection into the region will increase cloud cover Thursday evening with light precipitation possible after 2am. Precipitation should begin in the form of snow, but warm air advection will develop an elevated warm layer around 1470m that will change precipitation to sleet and freezing rain Friday morning. Surface temperatures will rise above freezing after 10am, ending the threat for icing.

We are only going to be looking at about .1" to .25" of water, which will be partitioned to around an inch of snow, a light coating of sleet, and a glaze of freezing rain on top. This, however, will be occuring right around the morning commute, so traffic may be slowed some Friday morning. Considering the timing, and if given a little more accumulation, we could have delayed openning on Friday.

Cheshire County Snow Totals

As anticipated, this storm delivered a tight snowfall gradient that aligned across Cheshire County. This resulted in amounts of generally 3-5 northwest of Keene, and then a rapid increase to 10 - 14 in the southeast corner of the county. Overall, a gradient of about an inch every two miles was present from Keene to Jaffrey and Peterborough. This is a little tighter than the gradient in the January 14, 2008 storm, but still does not challenge the infamous Blizzard of 2003 (President's Day II) on February 16-17, 2003 that gave Jaffrey 19.5" and Peterborough 21" while holding Swanzey to 8" and Marlow to 5".

Deformation zone

While moisture may lack toward the northwest of Worcester, it is made up for by strong low and mid level deformation that is offering decent banding on radar across W MA and S NH. The heaviest banding will likely stay just south and east of Keene, however from Rindge to Dublin could see 2"/hr rates for some time this morning. Keene will be on the edge, but still will continue to experience 1"/hr rates through the next few hours.

Closely monitoring the progression of the dryslot over NJ and NE PA right now. While it is progressing unimpeded into SE NY state, there is some backbuilding occurring over NYC and W CT, and this may keep snow going a little longer this morning.

As for totals, generally anticipating a range across Cheshire County of 5" in Walpole to 6" in Surry to 7" in Keene to 8" in Troy and 9" in Rindge.

Last minute changes

The NAM once again displays last minute inconsistencies. Lastest run (00z) brings the snowfall gradient much further east, with accumulations closer to 3-5 Walpole, 5-7 Keene, 7-9 Rindge. Radar trends supports a general drier forecast as well. I said in the past that this gradient would be a killer, and it looks like this will come true.

Snow overspreading the Mid Atlantic

Low pressure continues to organize and deepen along the Carolina coastline. It will move north northeast this evening and likely pass over the 40/70 benchmark tomorrow. Moisture convergence over southern New England will support a band of heavy snow with rates near 3" / hour possible along a line from just east of Hartford to Worcester to Nashua. These areas will see the highest accumulations, easily exceeding one foot, and approaching a foot and a half in some areas. Further east, mid level warming will change snow to sleet up to I95, with rain on the Cape. North and west of Worcester, snow amounts will decrease gradually then rapidly in a sharp cut off.

Latest model guidance has shifted this cutoff west some, with just the GFS continuing to have it across SW NH - however has been initializing the low too weak in the last few runs. Edge of the precipitation on radar is slightly west of progged. The 18z NAM brings W MA and S VT and SW NH into the action, introducing notably higher water equivalents of .8" to 1". At this point, I think any sharp gradient will be located more toward the VT/NY border, but it is still too close for comfort.

I will continue 6" - 10" for Keene, with closer to 4" - 8" for Walpole, and up to 8" - 12" for Rindge.

Timing:
Light snow will begin in Keene by 11pm this evening, and will fall heavily between 3am and 7am, at which point 4-6" could be on the ground. Snow will continue to fall moderately through 11am with an additional 2-4" and will taper off in the afternoon.

Major storm for E New England

After drier 18z runs yesterday, the 00z runs last night were much wetter, with a slower tracking system. The storm will pull in copious amounts of moisture from the Atlantic and Caribbean, with widespread 1+" water equivalents and up to 2" on the Cape. Consensus track is still about over the benchmark, passing within miles of the outer Cape. This will present ptype issues for the I95 corridor, but to the northwest of I95, out to Hartford to Worcester to Nashua, precipitation will be all snow and will pile up very quickly. After midnight, snow could fall at rates of 2" to 3" per hour in these areas. Jackpot totals will be between 1 and 2 feet. Further west, QPF drops off slowly, then quickly. The NAM has been particularly persistent in a rapid cutoff of precipitation. The 06z GFS moved a little toward this as well. The GFS ensembles continue to be the furthest northwest and wettest. It is a viable possibility that the gradient lies over Cheshire county, and Walpole gets 4" while Rindge gets 14". This makes the snowfall forecast for Keene very difficult. At this time, I think 6-10" is the best range for Keene, but there could be some surprises, for better or for worse.



This storm will be affecting the entire eastern seaboard. From North Carolina to Maine, widespread 6+" accumulations are likely.

18z GFS ensembles are wet

The 18z GEFs brings .75" to I91. A notable shift from the 12z run, as well as notably wetter than the 18z GFS operational run.

My current thinking:


Still looks like the jackpot lies from NE CT to Worcester to SE NH. These locations will likely see over a foot of snow. I did eliminate the 12+ area, and used 14 as an upper limit. Someone will likely see over 14, maybe even a few 20"s, but the timing of the dryslot does have me concerned with the the extent of QPF, so for now I'll play more conservative. To the southeast of Worcester, although QPF increases even more, approaching 2" on the cape, sleet becomes a concern, with rain or freezing rain possible on the Cape. To the northwest of Worcester, amounts will drop off with lower QPF. Sharp gradient from 8" down to 2" is likely. Right now, I think it may align about right along I91 north of the MA border, with totals to the west (Cheshire county) ranging from 4" to 10" west to east, and totals to the east (Windham county) between 2" and 4".

Shifts west, NAM onboard

The 12z ECM held pretty steady today. The 12z GFS came back from the east 06z run. And it took until the 18z run for the NAM to shift the 75 miles or so to get back in the pack. All in all, slowly reaching consensus. 12z ECM, 12z GFS, and 12z GGEM the favored models at this point.

18z GFS is a little concerning. It continues the trend of sliding a dry slot quickly northward, shutting precipitation off on the coast before daybreak Monday and reaching SNH by 7am. This lops about .25" off of total water equivalent across the region. We'll see where the rest of the guidance goes from this.

I want to take a look at the 18z GFS ensembles before posting my newest map, so I'll have that out probably around 6pm. But the general theme will be to shift the gradient NW from this morning. For Keene, we're looking at around .4" to .6" water equivalent, which with decent ratios translates to 5" to 9" of snow.

The area forecast discussion from the NWS in Taunton this evening highlights the uncertainty that still exists:

EXACT TRACK OF THE LOW AND MID LEVEL FEATURES AND FORCING MAY ALLOW THAT MAX AXIS OF SNOWFALL TO SHIFT WEST OR EAST A GOOD 50-75 MILES...ALLOWING IT TO OCCUR AS FAR EAST AS THE EASTERNMOST COAST OF MASSACHUSETTS TO AS FAR WEST AS THE CT RIVER VALLEY.

Sharp cut off

In the last 36 hours, we've had some convergence in the models, and yet notable disagreements remain.

There will be two separate events. The first one will be associated with weak waves of energy traveling south of the region tomorrow. This will spread a light snowfall across much of the region ranging from under an inch in Keene to 2-4 along I95.

The main event will be during the day on Monday. H5 low lifts north along the coast with dual jet convergence and surface low pressure strengthening just southeast of the region. Low level deformation will develop a band of heavy snow across southern New England. Generally looking at an 1" to 2" of water equivalent across SE MA, RI, and SE NH into S ME. Inside of I95 in New England, a notable portion of this could be sleet as the mid levels warm. However, strong high pressures to our north and west will secure cold air at the surface ensuring an all-wintry precip event for the entire region. Where precipitation stays all snow in SE New England, total accumulations could easily exceed a foot.

There is still considerable uncertainty with regards to the western extent of QPF. The nature of deformation banding will cause a very sharp gradient on the western edge. Literally, total snow accumulations will range from 2 to 12 inches in just 40 miles. The ECM has shifted west in its last two runs. The GFS was significantly west in its 00z run and then shifted back in the 06z run. The NAM is an eastern outlier with the cutoff actually along I95. SREF mean snowfall is also east. The HPC supports the western edge of guidance. Regardless, it is very likely that Keene will end up on the lighter side of the gradient. At this time I would say 3-5" for Keene. Given the right position of the gradient, it is quite possible that Walpole could get 1-2" while Rindge gets 6-9".

This is my current thinking, which is very much subject to changes.



By the way, for those with interests further south, portions of North Carolina through Virginia and Maryland could also be looking at accumulations reaching 12" from this storm tomorrow.

Complexity

Today's model guidance has trended toward supressing low pressure further south. The ECMWF, GFS, and ensembles show low pressure being forced south of the region, spreading moderate QPF across SE New England with light precipitation cutting off over western New England. This senario would deliver a couple inches of snow to NW MA, SW NH, with increasing amounts toward I95 where 4 to 7 is possible. Along and SE of I95, mixing with sleet and freezing rain will cut back on totals. 1040mb high pressure to our north will secure notable cold air damming across the region, ensuring a wintry senario for everyone. Should the low track shift back west, this would introduce greater snow totals in western New England and also present the potential for a formidable ice storm.

The NAM and SREF continue to display a track furthest NW, with a change to sleet backing even into SNH. QPF totals are much higher.

Many things consider: first - clearly there still is not a good consensus. Second, while the latest model runs are southeast, it should be noted that the ECMWF (the recognized voice of reason among weather models) has flip flopped three times ...in the last three runs. There is anything but consistency being displayed. Third, the one consensus is for very strong high pressure to our north and west. This should prevent a track any further west than crossing Cape Cod. As already said, this secures wintry precipitation for everyone.

Fourth - Most model guidance has switched to a two storm solution. The discussion above is for the first storm. The second low pressure that moves our way may track even further southeast. Considering this is only the first day that has shown two distinctly separate storms, and this type of situation is usually unstable, it's still possible that we move back to phased solution in which we have one main storm sometime Sunday - Sunday Night.

At this point, the timing is for the first storm to be centered on Sunday, and the second storm to move through on Monday- Monday Night.

Miller A storm potential Sunday

We've seen it shown a bunch of times this season, and it has yet to pan out. But here we are again, facing model guidance advertising a Miller A nor'easter for this upcoming weekend. This storm has been on the Euro for about 4 days now, and the GFS picked up on it a couple days after that.

The situation:

Energy will dive out of the Pacific northwest into the central plains by Friday with surface low pressure taking shape over the southeast.
A few senarios exist:
1) The storm stays completely detached from the northern stream, with a cutoff low over the southeast --- Major southeast snowstorm (possibly Mid Atlantic too), New England stays dry.
2) Northern stream shortwave phases with the southern stream and pulls an intense storm system up the east coast. Timing of the phasing would dictate wintry impact areas. An early phase = rain for most of the east coast with Appalacian snow. A later phase would result in a snowier solution for SNE.
3) Northern stream speeds up, but stays unphased. This results in two separate storms. The fast northern stream crushes western ridging resulting in the southern wave moving further north (impacting New England). Then the northern piece of energy moves in (some models have it triggering a coastal low).

Of the three possibilities, the likelihood of number one has diminished significantly in the last few days. Number three is plausible. Number two is most likely in my opinion. This would present precipitation type issues to most of the region. One thing to consider is the fact that we'll be looking at strong high pressure to our north and west following the storm on Friday, and going into the weekend storm. This should stop the weekend system from cutting north too quickly, so a Great Lakes cutter is unlikely. The other thing is that with the strong high pressure, ageostrophic northerly flow will create strong cold air damming. While, climatologically, it is difficult to get significant ice storms this late in the season due to the increased angle of the sun, it is not impossible.

Latest model guidance suggests that significant winter weather is likely to impact the northeast Saturday through Monday. The potential exists for this to be a longer duration event, with some areas picking up high QPF totals.

A nowcasting event

Still watching the latest model runs rolling in, however light snow is only an hour or two away, and it's time to watch radar, satellite, and the trend of current temperatures.

The GFS and NAM still fail to agree on the final result of the storm. The 00z and 06z GFS shifted toward the NAM's lower QPF with under an inch for all of SNE into CNE. Regardless, it's still advertising some very good omega of better than -20ub/s centered around 6 to 7pm. This is the timeframe that heavy snowfall rates of 1 to 2 inches could occur.

Meanwhile, the 12z NAM is rolling in. After displaying mediocre QPF for the last few days, suddenly, less than 12 hours before the height of the storm, it doubled QPF and is now in fact more robust than the GFS. It will be interesting to see what the 12z GFS comes in with. Otherwise, not much has changed with respect to the fact that CNE and NNE needs to prepare for a significant to major snow storm this evening.

For Keene -
Given continued model discrepencies, I'll hold at 6" to 9". More specifically, I expect 6" to 8" for lower elevations and 7" to 10" above 1000ft.

Heavy snow rates likely

The 12z and 18z GFS are extremely robust with the development of secondary low pressure just east of the region. By 00z Monday (7pm), H7 omega of greater than -20ub/s is indicated across SNH and CNH. Lapse rates are decent, so beyond boundary layer warmth, we still have H7 temperatures below -10C. This will provide for excellent snow growth. Frontogenic forcing will trigger snowfall rates of 1 to 3 inches per hour across interior CNH, and the possibility exists for some thunder. This is supported also by recent runs / trends in the ECMWF. A GFS solution delivers double digit snowfall to Cheshire County with the potential for more. Where the best banding sets up over NH (likely to our north and east), accumulations of up to 2 feet will be possible.

The NAM however continues to hold back on QPF. While the GFS spits out 1.25" to 1.5" across SNH, the NAM holds us to .5" to .75". Isn't it usually the other way around? For KeeneIn this case, the NAM is not consolidating the H5 vorticity, but keeps it in separate packets of energy. This is likely because of its higher resolution which sometimes plays tricks with the forecast. At the surface, this results in a slightly weaker secondary low, which consequently does not wrap moisture around into the region as efficiently as the GFS. It should be noted that SREF probabilities are still relatively low.

I am leaning toward the GFS with the rest of its model support, but still wary of the NAM. As for totals: Should be mainly rain inside of I95. East to west 1-4 inside of 495 and north of I90. 2-5 for SE NH. Interior RI and CT away from the coast should get 1-3. Northern higher terrain of CT could see 3-6. Expecting 3-6 for MA south of I90 and west of 495. Interior northern MA will see 4-8, possibly 5-10 for higher terrain of NW MA. SW NH (Cheshire and western Hillsborough) will see 5-10, possibly 12 over the higher terrain in northern Cheshire.

For Keene, I am holding with 6" - 9", but this has potential to be increased.

Significant snow storm tomorrow

Models coming in line now with secondary development occuring along the Mid Atlantic coast during the day Sunday, and moving across southeast New England Sunday evening. Most importantly is the latest prognosis from the 00z ECMWF which is on this track as well. The NAM is the furthest south and east with a track closer to the outer Cape. This would result in more snow in CT, RI, E MA, and SE NH, while heavier totals in SW NH and NW MA.

Precipitation type will be a big issue for much of SNE south of I90 and 495. While thickness criteria supports snow, boundary layer temperatures will be in the upper 30's for much of the storm. This will cause precipitation to be briefly wet snow changing to a mainly rain event.

North of Rt 2, this will be a mainly snow event, and especially into the higher terrain of SW NH, should stay all snow. However, we still will be looking at high water content and relatively low ratios. Regardless, with QPF around .7" to .9", snowfall totals will easier eclipse 4" here, with 8" quite possible.

Secondary low pressure will rapidly intensify Sunday evening, contributing to crashing heights and a flash freeze for eastern New England later Sunday night. Meanwhile, this will also present the best timeframe for what could be a period of heavy snow in western New England as strong frontogenic forcing becomes the focus for 2"/hr rates, and potentially blizzard conditions. Depending on far south the secondary low develops, SW NH could cash in on some of this as well, and this will need to be monitored very closely. In addition, total totals in the low 50's supports the potential for some thunder.

Further north and east, into N NH and ME, jackpot accumulations of 10 to 18 inches are possible through Monday.

For Keene -
Light snow will begin to fall by 10am as the intial clipper system slides east. Accumulations will stay light through the early afternoon, with probably just an inch or two by 3pm. Between 4pm and 8pm, the coastal low will be bombing to our east and this will trigger an expanding precipitation field with bands of heavy snow. Additonal snowfall of 4 to 5 inches is likely through the evening. Scattered snow showers will continue through the night and into Monday morning as upper level cyclonic flow remains entrenched over the region. Total accumulations of 6" to 9".