Hello all and welcome back - or perhaps I should wish a welcome back to myself after being absent from a post yesterday! Was busy with on-air coverage of severe thunderstorms and figured I did (hopefully) a good enough job explaining my thoughts on the event the day prior that I could let that severe weather assessment ride and let you enjoy the storms via the radar links on the right side of the page - hopefully that's exactly what many of you did!
What a difference those storms brought in! Large hail and damaging winds both were recorded, and the rain gladly lowered pollen levels for allergy sufferers in Southern New England. Some really interesting perspective from the drier and decidedly more allergenic northern half of New England, where one of our devoted weather observers in New Boston, NH, David Weaver, reports not only that the showers developed too far south for him, but also that his monthly rainfall total will be less than .5" if he doesn't record rain by the end of the month! That's remarkably dry for May, though it does seem as though we'll add soem drops to the rain collector on Saturday.
In the meantime, the air is fascinatingly dry. No really - it's fascinating! Morning balloon soundings indicate the convective temperature - the temperature the surface would have to reach to propel a parcel of air to the level of free convection (LFC) and generate non-dynamically induced lift - is running in the 70s and 80s for most of Central and Southern New England. Though there's a weak surface trough from coastal ME to eastern MA, the surface convergence along it likely isn't enough to produce much lift, so quite literally wall-to-wall sunshine will continue in almost all areas through afternoon, with the best chance of some fair weather clouds (small ones - cumulus humilus), and not a very good one at that, found across the far North Country.
Expect clear skies for most areas tonight, and the low dewpoints combined with a slackening wind will mean falling temperatures. Central, Western and Northern New England valleys will likely decouple and drop into the 30s, though dewpoints running in the 20s to around 30 may prohibit widespread frost and limit it to only the deeper valley communities. Nonetheless, a light frost or even a light freeze is possible, and I'd encourage those with tender plants in these lower elevation spots to cover them for the night.
A cool start turns around in a hurry on Thursday with plenty of strong sun - only three weeks from the summer solstice, now - and an increasing westerly wind that will downslope and aid in mixing. While this is the case for almost all of New England, the far North Country along the Canadian border, and northern ME, will find a moderate strength shortwave diving southeast over Quebec and ready to drive a surface cold front over the country line. The result will be scattered showers and probably some embedded thunder given the cold pool aloft. This is actually a pretty impressive shot of cold air, and though it's very diffuse later in the period, it actually *does* cool New England as a whole on Friday by only about 1 degree Celsius...but impressive for such a compact feature.
The Saturday system is one that will continue to be a challenge because both storm track and timing are up for grabs. That means low confidence forecast but we can make this easier on ourselves by asking the following questions: 1) When and where is the greatest warm and moist advection, and therefore greatest isentropic lift, on the synoptic scale? 2) Are there any other forcing mechanisms to reckon with? The answer to the first is Friday night into Saturday morning, and this is when I expect warm frontal rain. The answer to the second is "yes," a cold front and associated vorticity maximum. Storm track is still of concern, but no farther south than Central VT/NH/Srn ME seems to be indicated, which means we warm sector Central and Southern New England. It may not be the cleanest warm sector, but even breaks of sun would get these spots into the 70s and primed for strong Saturday afternoon thunder with the cold front, while Northern New England may not verify with highs in the 40s like the Operational GFS is showing, but there's certainly ample evidence for a very cool day when taking into consideration the thoughts on the intense little shot of Northern cool air on Friday that will have to be reversed.
This all sweeps through by Sunday, with downsloping flow. I still need to figure out why the statistical guidance is so dramatically far below the operational GFS guidance for Sunday...with the operational mid to upper 70s prog not even coming in as an outlier in the statistics! Given forecasted 850 mb temp, drying column and downsloping flow along with semi-occluded air, I favored the warmer solution.
Matt
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Posted by: guenstiges hotel | Tuesday, February 23, 2010 at 10:10 PM