Sunday, January 16, 2011

Final Pattern Change

Unfortunately, due to work and college, my time available to maintain the website has become limited. Rather than produce rushed, half-hearted forecasts, I've decided to retire the website. I've had a great time working on the site, and hearing the feedback, whether positive or negative (usually correlated to whether or not we had a snowday). Thanks everyone for your support and enthusiasm!

Special thanks to Nate Selvidio, who started the domain for me.

I've started a twitter account, which I should be able to update more readily without the time needed for other maintenance.

Sunday, November 21, 2010

Brief icy conditions possible tomorrow morning

As high pressure slides to our east tonight, winds will turn to the southeast, adding moisture to the low levels. A storm system currently moving through the Midwest will track into the Great Lakes region tonight, pushing a warm front toward the region from the southwest, providing a focus for lift and light precipitation.

Forecast soundings indicate a dry layer above the lower moist layer which is a classic indication of drizzle. Surface temperatures will start off in the 20's tonight, and rise into the low 30's by mid morning, allowing for freezing drizzle. As mid levels moisten a little more by noon, we may see brief light snow in central and northern NH, before flipping to rain.

No accumulation is expected, and this likely will not pose any issues. Just a heads up.

Looking ahead, adverse weather conditions appear likely for Thursday (Thanksgiving) night into Friday. Wintry precipitation is a distinct possibility, although there're still equal chances that it turns out as all rain. More details (and more confidence in those details) tomorrow.

Sunday, November 7, 2010

Wintry conditions likely tomorrow morning

Precipitation will be backing from east to west across the state this evening, with a rain/snow mix moving into SW NH by midnight. As the storm intensifies to our east, snow will increase in intensity, mixing with sleet and freezing rain Monday morning. Accumulations of 0.5" to 1" are likely for everyone with up to 2" above 1000'. In addition a light glaze of ice is possible. Surface temperatures will be below freezing, so be prepared for slippery conditions for the AM commute.

There is a slight chance of a delay tomorrow, dependent mainly on the freezing rain.

Wintry precip possible AM Monday

The trough associated with the storm system last week is now trapped along the east coast between ridging over the Midwest and the northwest Atlantic. As the trough sharpens, intense vorticity will slam against the ridge over the Atlantic, tilting the trough negative and cutting off as an upper low. This low will lift northward toward southern New England, with surface cyclogenesis over the western Atlantic. The surface disturbance will strengthen quickly with the upper level support and get pulled west toward the region.

With the upper low cut off from the mean flow, we won't have a source of cold air to the north, however we will benefit significantly from dynamic cooling as the low moves over head. Model guidance has trended west, placing the low near Boston by noon Monday. So this could be a situation where northern New England experiences rain showers while parts of central Massachusetts and southern New Hampshire are seeing moderate snow!

Based on the latest model trends, the potential does exist for some accumulation over the specified areas, including Keene, Monday morning. Certainly something to keep an eye on.

Saturday, October 30, 2010

Significant storm likely Thursday

In the last few days, model guidance has been keying in on a potential nor'easter on Thursday (11/4). This storm potential coincides with a large scale pattern shift early next week.

General overview:

The strong MJO wave that traversed Indonesia in the last two weeks has been responsible for a surge in tropical cyclone activity in the Western Pacific. The most recent of which is Typhoon Chaba which reached category 4 intensity near 25N, and has since recurved just east of Japan. Upper level divergence and latent heat release has pumped up ridging ahead of the cyclone, triggering a wave train across the Pacific into North America.

In addition, the AAM relative maximum is over, with a strong negative tendency appearing in the northern subtropics. This will propagate northward, having the effect of temporarily weakening the westerlies over the north Pacific. As a result, perturbations in the jet can become more amplified.

Induced ridging over the West Pacific will shift into the central Pacific, supported by La Nina's weakened hadley cell. Downstream, this will amplify a trough over the Gulf of Alaska and ridge over western North America into early next week, with the positive EPO becoming more neutral. Between the ridging to our west and departing storm system on Monday, strong cold air advection will keep temperatures around 5 to 10 degrees below normal over the region.

On Tuesday, model guidance shows some energy from the GOA trough ejecting over top of the ridge into the Northwest Territories. This energy subsequently carves out a trough over central Canada which then digs into the Great Lakes region Wednesday Night and triggers surface cyclogenesis south of New England.


Difficulty in the forecast arises with the interaction of a number of individual short waves.

First to the show is a piece of energy that gets trapped underneath the developing ridge early in the week. Model guidance agrees on developing a weak cut off low over Texas, and most agree on the ridge shoving the low south into the Gulf of Mexico, and not being a significant player. However, the GGEM has been persistent in shifting this low east and phasing it with the trough digging into the eastern US, to make one extremely amplified disturbance displaced to the south. Ultimately this would mean a very warm moist southeast flow over the region with low pressure tracking inland over New York. This solution is being discounted ... for now. The latest run of the ECMWF is leaning in this direction, so unfortunately it cannot be completely ignored.

Next we must deal with the energy within the trough. The operational GFS continues to show very strung out vorticity, with surface cyclogenesis delayed until northern stream phasing occurs. As a result, low pressure develops east of the region and moves northward into Nova Scotia, giving New England only showery precipitation. This solution has little support, and may be less likely than the GGEM.

The ECMWF and UKMET on the other hand phases earlier, resulting in a more amplified solution, with surface low pressure developing east of Delmarva and moving over SE New England. The GFS ensemble mean is also closer to this solution versus the operational GFS, though with notable spread. Both the ECMWF and UKMET show a very dynamic situation with cooling temperatures aloft despite strong southerly flow. This would suggest heavy precipitation rates, falling as snow for much of the interior. In addition, this storm may have some tropical connection depending on the interaction of the trough with Tomas. So moisture will be plentiful.

The possibility does exist for notable snowfall over the interior higher elevations, dependent on the track of the nor'easter. One way or another, we will have a significant storm on our hands this week.

Saturday, July 17, 2010

A preliminary look at Winter 2010-11

The past 9 months has featured a plethora of broken records from the Pacific to the Atlantic to the Arctic, stratosphere, and the Sun. Because of this culmination of meteorological phenomena, a set of analog years could not possibly come close to representing the current state of the atmosphere. A more qualitative approach is better this year.

Reviewing what got us where we are now: Warm waters in the western Pacific steadily collected near the dateline by three significant westerly wind bursts through the autumn and winter months. However, these westerly winds never proceded far into the eastern Pacific where easterly trade winds maintained strength. The Peru current fed colder waters into the extreme eastern Pacific, forcing a west-based El Nino through the winter. A strong west-based El Nino is rare because normally the SST anomalies force a circulation with positive feedback pushing warm waters into the eastern Pacific. In the winter of 2009-10, the ONI hit 1.8C for NDJ, and 1.7C for DJF, indicating a strong warm episode, but the highest SSTAs were locked in the Nino4 region (near the dateline). This season featured the highest monthly Nino4 SSTs ever recorded for October through February. The strong El Nino powered an active subtropical jet that contributed to the multiple historic snow storms in the Mid Atlantic and Southeast US.

After the last westerly wind burst in February, the thermocline began to rise in the central Pacific, with a layer of cool subsurface anomalies developing. The persistent tropical forcing near the dateline subsided in late March, and the MJO became active through the beginning of May. Trade winds reintensified and by the start of May the central Pacific had cooled substantially. Subsurface anomalies began to affect the far eastern Pacific in the middle of May and these surface anomalies subsequently expanded west into the Nino 3 region. By this time, tropical forcing had made a complete 180, with persistent positive OLR and 200mb convergence anomalies near the dateline and negative anomalies in the Indian Ocean and Atlantic.

During the last four weeks, the equatorial Pacific has continued to cool east of 150E. Along and to the west of 150E SSTs have warmed with notable postive anomalies developing. After heat content in the equatorial Pacific steadily decreased between March and early June, it leveled off through early July, though there is new cooling near 140W. What is notable is that higher OHC is slowly expanding eastward in the far western Pacific, near Indonesia. Overall, OHC anomalies are significantly different from 1998 (a proposed analog). The strong west based El Nino last winter compared to the huge east based El Nino of 1997-8 makes the subsequent states of the Pacific two entirely different beasts. In '98 the EPac maintained above normal heat content through October and there was no push of warmer waters in the WPac and consequently that La Nina hung around for a couple more winters.

This year, there is a very clear west push of +OHC near 5N to 10N which is already being reflected in the far WPac at the equator. We should see warming continue in the western Pacific and extend into the central Pacific by late autumn. La Nina will stick around this winter, but it will be EAST based (Nino3 anomaly < Nino4 anomaly) and the DJF ONI will be around -0.8C to -1C. This is also supported by various forecast model ensembles and roll forward methods. Warmest waters in the world right now are near Indonesia. The only change through autumn is perhaps expand east somewhat. This will enhance low level convergence near 150E sustaining MJO waves likely into phase 6 before collapsing. The few times this winter that the MJO survives into the western and central Pacific will be when blocking is most favorable. This is most likely in late December (also supported by La Nina climatology) and February as the La Nina begins to wane.

In the stratosphere, the QBO has set lowest monthly records for the last few months. Westerlies are now descending pretty quickly and will probably reach 30mb by late December. Ozone took a huge hit from high levels of water vapor associated with the strong El Nino, plus supporting the record breaking negative AO. This can be seen this summer manifested through a persistent strongly positive AAO due to the lack of ozone being transported in the BDC. Again the rapid developing La Nina may come to the rescue as water vapor diminishes significantly near the equator and ozone levels recover, which lends support to a possible stratospheric warming. The positive AAO this summer also correlates with above normal arctic heights / -AO next winter. The -QBO easterlies may hang on just long enough to support a SSW in mid to late November, that will significantly disturb the polar vortex. In the last few months, a -PDO SST signature has become better defined in the north Pacific with warm waters stretching across the basin near 40N. The La Nina pattern will certainly help maintain this general feature. The warm waters in the northwest Pacific will not be an easy road for the PV to take, which will more likely end up in the western hemisphere. I like the chances of colder air making it into the CONUS by early December; with moderation toward the middle of December.

In association with the decaying El Nino, global AAM took a plunge in early March. In early June, the AAM bottomed out again, with the 4th strongest GWO in phases 0.5 - 2.5 for the April-August period on record. This boils down to strengthening trade winds in the Pacific lending support to La Nina through positive feedback. Expect the GWO to continue to be displaced toward the La Nina attractor through the autumn and early winter. With this in mind and the prospect of ridging across the north Pacific, there will be multiple strong frictional torque events this winter. Three to five days prior to the event, there will be a significant storm threat from the midwest into the Ohio Valley and northeast as east based -NAO / Atlantic ridge retrogrades west, and SE ridge breaks down. Could see this happen around December 25th, with -NAO pattern and arctic air settling into the Great Lakes and New England region in the end of December / beginning of January, with continued storm threats through the first half of the month. The pattern will lean back toward a La Nina state by the beginning of February, with overrunning snow/ice threats through the first few weeks. Another torque event by the end of the month will bring winter back into the region into March, though with more La Nina influence.

The issue with looking for arctic outbreaks is the enormous heat release from the El Nino last winter. It's been an issue with strong El Ninos in the past, and especially last winter because the anomaly was further west with SST totals the highest of any El Nino. At least the quick transition to La Nina this summer will likely help shift the pattern to a colder arctic through the autumn, with on average stronger mid latitude ridging (as we've already seen this summer) and +AO/+NAO developing for a few months. This will allow colder air and hopefully a recovery in arctic sea ice and snow cover in preparation for mid latitude winter.

In the Atlantic, record high SSTs have been in place since January. The anomaly distribution matched very well with the record breaking -NAO last winter. The vast majority of the tropical and subtropical Atlantic has been well above normal. Every month so far this year has set a new record TNA, and in fact the May TNA set the all time record. The April and May AMO were the highest since 1878 (keep in mind the trend is removed). The June AMO dropped slightly with 1998 taking the lead (oh no! that year again). Also in the same camp is 2005 and 1995. And like those two years, it is no secret that a very active hurricane season is expected this year.

A weak -NAO has continued into the start of the summer, and the signature SSTA tripole in the Atlantic is expected to continue into the Autumn months. The cold midlatitude anomaly is fairly limited in area and amplitude, and is displaced east of the standard location. Warm anomalies in the Gulf Stream are much more robust than other tripole years. One thought to this point is enhanced baroclinicity helping intensify and guide coastal storms in the late autumn and winter. The tripole will help assist blocking patterns to some extent, though the main theme of the 2010-11 winter will likely be an Atlantic ridge pattern. This will be broken up leading into frictional torque events, which will help set up periods of -NAO blocking.

Most of all, I expect this winter to be remembered for abnormal storminess. I expect huge precipitation anomalies from the Ohio Valley into the northern Mid Atlantic and New England. Whether cold air is present will dictate snow, and it shouldn't be too difficult to tap into some semi-arctic air for these systems. General southeast ridge pattern will dominate otherwise, extending into the northeast in early February. Other warm areas will be southern and central plains into the Rockies, with below normal temperatures in the Pacific northwest.

Friday, July 2, 2010

Invest 95L near Florida panhandle

As we enter the month of July, a big question on the mind of meteorologists is, how active will the tropics be?

Atlantic ocean sea surface temperatures continue to be well above normal, even above 2005 levels. And as a la nina takes over in the equatorial Pacific, westerly shear is overall below normal across the Atlantic. In addition, intrusions of dry air from the Sahara have been limited. These three main factors are all positive for a very active, possibly historic hurricane season. This is nothing new; there's been a strong consensus since the spring for such a possibility.

June is officially the first month of the Atlantic hurricane season and on average only produces a tropical cyclone every few years. Historically it has practically zero correlation to the total season activity. June 2010 was relatively active however, producing Hurricane Alex, the strongest June hurricane in the Atlantic since 1944.

Now onto July, the correlation between monthly activity and total season activity becomes a little stronger. Just think back to 2005, when a record breaking five tropical storms formed, three of which were hurricanes, with two major hurricanes. These numbers obviously gave some foreshadowing to the rest of the record breaking season.

Currently in the Atlantic, there are no monster tropical systems in the making, just a few disturbances to discuss. First and foremost, we have a new invest, 95L just south of the Florida panhandle near 29N/85W. The low is left behind from the trough currently moving offshore (see "heat wave" post), and is nontropical at this time. The is moderate shear and dry air affecting the system, with any thunderstorm activity confined to the south of the circulation. Model guidance brings 95L west and then turns it northwest into Louisiana as a shortwave dives into Texas. At this time, no significant tropical development is expected, but periods of rain and some gusty winds may impact the central Gulf coast within the next four days.

One other noteworthy disturbance is located in the south central Caribbean near 12N/75W. Scattered moderate showers and thunderstorms have been associated with a weak circulation, moving slowly westward. The NAM organizes this tropical wave in the western Caribbean over the next three days. Given a moist, low shear environment, the potential does exist for some slow development. If still in tact, the wave could move into the Bay of Campeche by next weekend. The BoC is often a breeding ground for quick developing tropical storms, so this wave may need to be watched.

Lastly, there is moderate convection associated with a tropical wave near 10N/50W. Most of the convection is within the ITCZ, and no short term development is expected.

Elsewhere across the Atlantic, a strong upper level trough can be seen on water vapor imagery along 60W. This trough is generating strong vertical shear from the NE Caribbean into the central Atlantic.