CRISP FALL AIR TO LEAD TO NORTHERN FROST AND, FOR SOME, FREEZE

Yesterday's weather depended heavily upon location.  For some, the gusty pre-frontal winds and squall line both showed up, on cue, while for others, nothing but a band of rain arrived.  This was heavily dependent upon where warm air was able to get in - the farther north one was, the tougher it was to get the warm front through, and the farther west one was, the pre-frontal trough that ended up resulting in the squall line for central and eastern Southern New England brought rain to western areas early in the day, cooling the boundary layer and creating a shallow but stable cooler surface dome of air that was not penetrated before frontal passage during the afternoon/evening, which, for some, brought the most significant winds of the day.  For others, damage was done from the squall line.

Nonetheless, it's gone now.  Our attention turns to the very dry and cool airmass that's still advecting into New England on a northwest wind ahead of the anticyclone migrating east out of the Great Lakes.  This high will crest over New England late Wednesday night and early Thursday morning.  What this means is that the fair weather cumulus humilus and cumulus mediocris that are developing Wednesday afternoon will melt away as diurnal heating wanes, leaving clear skies.  The winds will quiet quickly with the approach of the high pressure center, and the atmosphere will decouple, leading to rapid temperature drops overnight.  Dewpoints are running in the 40s, but are still dropping in Northern New England and will be in the 30s overnight.  Temps will correspond to these dewpoint values, and frost is likely for most of VT, Northern NH and Central/Northern ME.  Hard freeze seems likely in the deeper valleys of the Greens, Whites and valleys of the Maine mountains.  There also will be a few patches of valley fog, and I suppose a touch of freezing fog is possible in the deepest valleys of Northern NH!

One thing we consider in circumstances like this is whether we need to worry about an isolated dust devil when the temperature warms in the morning.  The principle here is that very dry air expected to warm rapidly will create thermals rapidly early in the day, and the differential heating early in the day can enhance these thermals relative to their surroundings, creating pockets of increased buoyancy.  The high relative humidity where some fog is observed early makes these much less likely, but fog-free areas may warm quickly enough that an isolated dust devil can't be ruled out - though these are extremely difficult to predict in an operational setting.

As the anticyclone drifts east, the corresponding return flow will begin across New England, but only very lightly out of the west-southwest.  This will lead to some weak warm advection, and with the wind speed less than Wednesday, the feel on the body will be more than just a few degrees warmer Thursday afternoon.  Sunshine should be abundant with a deep dry column.

The upcoming weekend features a fast, mostly zonal flow across the Northern Tier of the country.  This means a series of fast-moving shortwaves embedded in the flow that will ride over New England Friday, Saturday and Sunday.  Friday's disturbance is late enough, and the moisture return slow enough, that showers are unlikely until evening or Friday night, as warm and moist advection increases, especially in the lowest 10K feet of the atmosphere.  This is likely to result in building, increasing Friday afternoon/evening clouds leading to the shower threat.  The NMM is developing a wave of low pressure with a solid inch of precipitation Friday night.  The GFS is far less pronounced.  The GGEM and many of its Ensemble members continue to favor a prolonged stretch of at times heavy rain this weekend, as moisture from the southern/central US and even some of Ike become entrained into the flow.  I have doubts on both the NMM solution of digging the northern stream shortwave, and with the solution of getting some of Ike involved, at least enough to make a difference for most of the weekend.  Therefore, I like the GFS solution best, which brings some showers through Friday night (lines up well with the warm advection), but keeps the shortwave a bit flatter.  Then, there does seem to be good agreement on the timing of the shortwaves among the guidance, even though the QPF forecasts are different.  So, in the end, I think we're looking at a few showers Fri eve/night, then again with the next shortwave Sat night into early Sunday morning, then maybe again Sunday night.  Of course, this timing is extremely optimistic for the weekend and the meteorologically inclined know that such fast and nearly zonal flow can change timing, but it seems like the best forecast right now, leaving an overall optimistic weekend.  More moisture seems to feed in with each shortwave, but that also seems likely to be cutoff for the middle of the week by a large anticyclone dropping into the midwest and ohio valley.  As that pulls out, the rest of Ike would be open to head up to the Northeast by the end of next week.

Now let's see what reality has planned!!

Matt

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