AS DEEP SUMMER WARMTH RETURNS, ATMOSPHERE MOISTURE-LOADS IN WHAT MAY BE AN EARLY WEEKEND DUMPING OF POTENTIALLY FLOODING RAINS...
Welcome to the weather blog - a regular Monday through Friday discussion of the weather! While the discussions usually will only come on days I'm working, I'll issue special updates when the weather warrants. I will always post to let you know when no discussion is expected if I'm away on vacation, etc. - if no update is here and no info is available, that likely means the server has temporarily gone on the fritz and I will update as soon as technically possible. You'll find a quick weather synopsis linked to the daily forecast at the top of the page, a general non-technical weather summary below, and when available (most days) a detailed technical meteorological discussion will follow by mid-afternoon. My email is firstname.lastname@example.org. This blog is for you, so I hope you enjoy it! -Matt Noyes
General Weather Summary:
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The deep summer warmth and humidity is returning to New England thousands of feet above our heads Wednesday morning, and will continue to make its push northeast through the day. At the surface, the transition isn't quite so simple, however, as a cool and somewhat less humid airmass lingers in Eastern New England, and the clash of airmasses resulted in plenty of low altitude clouds and areas of dense fog Wednesday morning. With winds gradually turning to blow from the south - but still out of the southeast along our coastlines - not only will warmer and more humid air be causing this collision of airmasses, but winds will still be blowing in off the cool ocean waters. The result should be for stubborn clouds amidst breaks of sunshine on places like Cape Cod and the coast of Maine, and cool temperatures reaching only the 60s in both of these locales, though fog will continue to slowly lift. Along the remainder of New England's coastlines, this slight ocean component will keep the sun hidden a bit longer than inland locales, but sun should certainly still emerge with temperatures in the 70s, and where ocean influence is less pervasive through the interior, we'll warm into the 80s. In fact, enough warmth is in place across Northwestern New England that the Champlain Valley should exceed 90 degrees Wednesday afternoon! In these warmer areas of Northwestern New England, however - from Vermont into Northwestern New Hampshire - the abundance of warmth and humidity will make the atmosphere ripe for showers and thunderstorms to develop with an upper level disturbance passing through Wednesday afternoon and evening.
That same upper level disturbance will set into motion a cold front at ground level, sagging the front southward out of Canada overnight Wednesday night to keep scattered showers and thunder going in Northern New England. Elsewhere, a fairly quiet albeit mild and muggy night is expected with a few areas of fog possible in valleys of Central and Southern New England.
The southward sagging cold front will mark the leading edge to cooler air locked over Canada, and this new air will drain southward across the Canadian border on Thursday, though most of the remainder of New England will find deep summer warmth in place, with temperatures climbing well into the 80s and a few spots perhaps even exceeding 90 degrees! With the aforementioned cool front draped across New England, however, it won't take much to generate widespread showers and thunderstorms, and a weak wave of low pressure developing along this front across New York State will be just the type of mechanism needed to focus those thunderstorms. Some Thursday afternoon storms are likely to pack a punch with regard to heavy rains, frequent lightning and localized damaging wind gusts. Overnight Thursday night this cold front will slow over Southern New England, keeping scattered showers in the forecast.
Bigger problems may be in the offing for Friday and Saturday, however, as signs are strong that this cold front will slow to a crawl as attempts to move south of New England Friday into Saturday. The reason this is a potential problem is because a new wave of low pressure will be moving northeast out of the Tennessee Valley, packed with Gulf of Mexico moisture and loaded with the warm and humid summer air that's been entrenched over the Eastern U.S. the past several days. We'll add to this a surface and low level flow out of the Bahamas, and that means a tremendous amount of available moisture. Meanwhile, the aforementioned cold front will be slowing, but cool air will be moving into Northern New England behind it, and this setup of tropical air, deep moisture, and cool air all colliding is a recipe for heavy rainfall. In fact, those of you who follow this blog know that this is a similar setup at the surface to what we saw leading into the major flooding of two weeks ago. Having said that, the upper level pattern is quite different this time around - and quite more progressive. Therefore, heavy bands of rain are not expected to linger nearly as long, less moisture will be available aloft, and we're not looking at a foot and a half of rain. But even a shot of 3"-6", which is a possibility, would cause renewed flooding concerns, so I'm going to be watching this system very carefully. If we can get the cold front to stall just a bit farther offshore, the heaviest rainfall would come down across the waters south of New England instead, so that would be a possible but at this point not very likely saving grace.
As for your weekend, I wouldn't get excited for any rapid clearing behind this rain-maker, but there is certainly some hope to salvage part of Sunday, in particular. After periods of rain linger on Saturday, we should swing winds around from the northwest behind the departing storm and its counter-clockwise circulation, which would favor some drying on Sunday. Still, with moisture wrapping around the back of the storm, at least some showers across New England - and especially Northern and Eastern areas - would be likely.
Matt's Technical Meteorological Discussion: Updated Wednesday, May 31 at 1:05 PM
I'll skip a rehashing of yesterday's discussion to set the scene and just build upon it, so if you missed yesterday's feel free to look back through the archives/recent posts that can be accessed through the sidebar.
Break in the action today comes courtesy of NVA induced subsidence inversion aided by noctural inversion invoked by cool and dry low level airmass. Nontheless, this inversion has mixed out thanks to strong diurnal effects through most locales, tho an onshore component to the surface wind has made the inversion slow to break at south-facing coastal locales and as a result the boundary layer moisture and low clouds have been slow to break up and burn off. I expect this sluggishness to last into afternoon as the onshore component remains in place, with the strongest effects in SE CT and the coast of ME. Elsewhere, atmosphere will continue to mix and after overnight and continuing warm advection aloft this means we're mixing deep summer warmth back into the boundary layer and temps will continue to respond. Tropical air still in place Champlain Valley and Northern VT...heading for Upper Valley...and these locales will find warmest max temps, highest dewpoints, and this combo will breed sfc based CAPE values in excess of 3000 J/kg and LIs to -6. This instability, combined with falling heights in advance of the next shortwave which will still be across Southern Ontario but will be the impetus for convective development in Vermont thru the Upper Valley of VT/NH this afternoon and eve.
Overnight as the sfc cold front sags south, showers and thunder are likely to sag south and expand E into Maine. This will be aided by warm and moist advection continuing in the low levels and at the surface, which may allow for fog in valleys but wind should stay up enough to prevent fog in exposed locales. Nonetheless, the onslaught of warmth and moisture overnight ahead of the vort lobe riding ahead of the main shortwave and a resultant prefrontal surface trough will be enough to maintain and in fact likely strengthen convective activity over NY and running into VT and Nrn NH before weakening while expanding E into ME.
Thu will be an active convective day across NewEng thanks to the combo of falling heights, cyclonic vorticity advection and plentiful tropical moisture. After a thermal profile that would support widespread temps nearing 90 in its 00Z run, the 12Z NAM has backed off given increased convection and given the combo of falling heights and a nearby cold front that will feed thunderstorms. The truth likely lay in the middle and that's the road I'll take. As for the convection, I think CAPE will actually be higher than the 1000-1500 J/kg forecasted by guidance because cloud cover is likely to be less than progged at least until early afternoon, and even between convection heating should continue thru the afternoon. This will raise the potential for damaging downburst winds but winds are relatively light under the low level trof which doesn't lend any support beyond the convective available potential energy, and with precipitable water values approaching 2" I actually would have to think that relatively slow moving storms featuring torrential rains capable of localized flash flooding and plentiful cloud to ground strikes would be the more widespread concern.
I have significant concerns for rainfall totals from Thu thru Sun across Central and Southern NewEng, and though there is still uncertainty that exists with regard to placement and intensity of heaviest bands of precip, I'm not encouraged by what I'm seeing - that is, conditions do appear favorable for several inches of rain in Central and especially Southern NewEng. Flash flood guidance is 3-4" in 24 hours and given the available amount of moisture thanks to a tap from the Gulf of Mexico, a surface flow from the Bahamas, and a trof that begins positively tilted but goes neutral toward negative on Saturday, I think we have bands of rain that begin as thunderstorms Thu, then fluctuate in intensity but become focused heavy rain bands with multiple waves along the slowing frontal boundary Fri and Sat and with cool air that means business heading south from Canada behind the front we not only have great surface convergence but also some differential advection which is always great for persistent heavy rain bands. 3"-6" seems like a safe forecast right now though that may have to be upped when taking Thu storm production into account. While this is far shy of the event two weeks ago, it's still a significant slug of rain that will raise flooding concerns. We will delve deeper into this tomorrow, as data becomes more readily available, the atmospheric structure better progged, and I have more time to expand.