It's been awhile since I've composed one of these, eh? As mentioned in some previous Ride Home posts, it's been a busy stretch, but I continue to work to get as much back on track as possible. With Danielle Niles, my sidekick and nearly-twin (though she isn't nearly as old as this aging meteorologist), taking this week off, yours truly will be flying solo and it's important to get well spun-up this eve. Creating some graphics for tomorrow's shows from home, but first and foremost is getting a solid handle on the weather pattern.
An initial look at the national weather shows severe weather erupting through the nation's midsection, indicative of plentiful upper level energy crossing the country. Convective cells embedded in Pacific Northwest rain have prompted a tornado warning in southeastern Washington State, while deep thunderstorms flare off the Florida coast. Meanwhile, closer to home, two lines of downpours with convective elements race across the Northeastern United States - one across Northern New England, and the other racing east across the Southern Tier of New York. Satellite imagery fills in a few more gaps - a cloud mass traversing the Ohio Valley appears to have eyes on New England, and though the deep northern energy spawning Western and Central United States severe weather looks ready to move northeast into Canada, the active southern stream carries shortwaves that will interact with a front draped from the Ohio Valley to the Northeast. Nailing the location of this front, and timing of shortwaves interacting with it, will be key to the forecast each day this week.
The longwave pattern for the upcoming week should feature a weakening of the pronounced trough over the Western United States, while an upper low and associated trough linger east of New England, over Newfoundland. Though the western trough may weaken in the days to come, the mean trough position remains there and east of New England, leaving weak ridging over the Eastern Great Lakes. New England, meanwhile, remains on the cusp of the confluent flow bridging weak ridging to our west, and the upper low to our east. This will allow for a wavering frontal boundary with repetitive shortwaves - affirming what the current analysis described above hints at.
In fact, 500 mb height and vorticity progs indicate a shortwave each day, by in large, but there is tremendous disagreement among guidance solutions after 60 hours on the evolution of, intensity and timing of shortwaves. Most of this disagreement is owing to convective feedback issues and the differing convective parameterization schemes that lead to variable handling of thunderstorms, and how they feedback on atmospheric pattern evolution. In the short range, there is good agreement that the shortwave prompting showers and downpours Sunday evening will continue moving east-southeast across New England, prompting showers across Southern New England overnight, as dynamic lift shifts across the region. Behind the showers, clearing skies will move into the North, but this will also prompt fog and low-altitude cloud development for those areas where Sunday evening rain left abundant boundary layer moisture.
Though showers will be done Monday morning in Southern New England as drier air advects southward in the mid-levels, plentiful moisture at and below 850 mb will leave a gray stratus deck early, that will slowly break up from north to south as dry air aloft erodes the deck. Some of Western CT - and, indeed, most of CT - may stay mostly cloudy throughout Monday, as the Nutmeg State will be on the eastern side of a strengthening baroclinic zone over Upstate New York, associated with the warm and moist advection ahead of a potent shortwave and upper low in the Upper Great Lakes, and its clash with the confluent cyclonic flow east of New England. This leaves shortwave ridging over Eastern New York, meaning upper level diffluence over Upstate NY, and that will promote rain in that area later Monday. In fact, it's the same shortwave that touched off Sunday evening thunderstorms in the Central Plains that will shoot east in the fast Westerly winds, providing a shot of warm and moist advection to New England on Monday night, allowing rain to move east out of New York State as a 45-55 kt mid-level jet punches across the state line. With a surface low on the eastern tip of the Upper Peninsula of Michigan, under the upper low, a triple point low over Upstate New York beneath the upper level diffluence, and a bubble high over the Eastern Gulf of Maine feeding cool air west into New England, this provides not only excellent directional and speed convergence throughout the lower several thousand feet of the atmosphere, but also a situation of differential advection on Tuesday, with cold advection at the surface and meaningful warm and moist advection aloft. Add to this the fact that the surface wind is bringing a long fetch across Atlantic waters, and this only increases available moisture to the atmosphere, with the differential advection providing extremely effective processes for efficient precipitation production on Tuesday. As for precipitation amounts, highest amounts of around or over an inch seem most likely in Southern New England, with lesser amounts but still a meaningful .50-1.0" of rain in Northern and Central New England.
Though some dry air will become ingested into the cyclonic swirl of the upper low on Wednesday, allowing for some sunshine in many areas, the amount of energy involved with the upper low is quite impressive, and dependent upon timing of the upper level cold pool, thunderstorms with vigorous updrafts and hail are possible in especially Northern ME (the new severe weather capitol of New England, it seems) on Wednesday afternoon. Will have to see how this plays out, but cold pool does appear impressive enough to trigger convection, even though boundary layer is quite cool. Incidentally, for those wondering why Aroostook County, in the northern tip of Maine, has seen two tornado touchdowns (three if you include the skipping tornado of last weekend as two...but it was one storm with a skipping tornado, it seems) and a sleu of severe weather, it's certainly not the abundance of warmth and humidity, as normally is viewed as necessary for fuel in severe weather events. Rather, it's the presence of extreme COLD air aloft, several thousand feet above the ground, much like what we're looking at for a potential on Wednesday. It's this cold air aloft - abnormally cold air - that provides the difference between relatively warm surface air (even if only 55-60 degrees, it's warm compared to up high in the sky) and cold air above that is termed "instability," and promotes deep vertical cloud growth that produce thunderstorms.
Differences become quite pronounced by Thursday. Operational NMM, GFS and GGEM (global Canadian) forecasts indicate a break in the action between shortwaves, with New England in confluent and still cyclonic flow. The GFS, Canadian and ECMWF Ensembles all indicate significant precipitation possible. In essence, a blend of both solutions seem reasonable - rather close, perhaps, to the Operational ECMWF. That is, the height and vorticity pattern support a break in the action with drier and cool air in place as the upper low pulls east. But that also means that as the next shortwave approaches, there is going to have to be a very substantial slam of warm and moist advection that takes place, in an effort to displace the anomalously cool air that followed the midweek upper low. This should mean a return of unsettled weather later Thursday lasting into Friday, and once again we may find a decent shot of precipitation given the amount of isentropic lift that takes place. How much rain? Though it's early to say, the Ensemble solutions indicate a widespread inch of rain is possible here.
So, the potential exists for a couple of inches of rain (at least) for some parts of the region this week, especially in Southern New England. Admittedly, one can find reason to be a bit gunshy on this, considering last week's front remained stalled just south of New England, delivering heavy rain to the Mid-Atlantic, and this can raise concerns that the front may do the same thing again this week. That seems unlikely, however, given that the jet stream is lifting north, and therefore this should actually increase confidence, not decrease it. This same pattern, just slightly farther south, produced a couple of 1.5"+ rainfall events in New Jersey last week, and may do the same in New England this week. As for any severe weather threat like was seen in the Mid-Atlantic last week, that's not impossible, especially as the aforementioned cold pools drift overhead, but also seems far less likely here in New England, given our proximity to cool bubbles of Canadian high pressure that severely retard any pockets of instability from developing, and rather increase the potential for the precipitation producing differential advection. Flooding? Not likely. At least not in the first event, and probably not in the second. Antecedent dry conditions have 12 and 24 hour flash flood guidance over 3" right now.
Next weekend? Early to say in such an uncertain pattern, but I'd have to say that the combination of a developing southerly boundary layer flow with a broad trough aloft would mean near or slightly warmer than normal temperatures with increasing humidity and a chance of thunderstorms - possibly strong - each day. We'll see how close to reality that becomes.
Enjoy your week - I'll hope to update here with either written discussions or more likely some Ride Home video blog updates. I'll also get some General Weather Summaries out on WeatherNewEngland.com, our NECN Weather Team website. I'm up in about 6 hours but would like to crank a few graphics out for tomorrow's show before bed, so may be up for a couple more hours. Either way, see you on air in the morning.
Matt
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