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The Ride Home - Tuesday, June 23, 2009 - Episode #22

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It's been wet!  The dog is loving it, most humans are hating it.  The pattern should break after our huge, slow, upper-level low departs, bringing a more progressive, brighter, milder, though still active pattern for the next few weeks.

Sunday evening prospectus, June 21, 2009

A relatively quick prospectus tonight to outline what's developing this week.

The longwave pattern features our stalled upper level low south of New England that will weaken gradually, producing showers of a more scattered nature as it fills, then finally being swept northeast as rising heights move across the Northeast.  This allows for a long-range pattern that will feature much higher average 500 mb heights across the Northeastern United States, indicating the average temperature will be noteably higher for the start of July, though it's interesting to note that the mean trough position remains over Eastern Canada, the Eastern Great Lakes and New England, varying in longitude from time to time, but producing a pattern that should bring thunderstorms every few days, and the potential for shots of cooler Canadian air behind each cold front, though not as cool as the recent spade of chilly weather, with average high temperatures near or slightly above normal over the period from late this week through the first week of July, with above normal temperatures likely during the first half of that timeframe.

In the shorter range, the most immediate effect on our weather is clearly the storm center stalled to our south.  Surface and low-level convergence bands have been focusing ocean moisture to produce bands of steady rain that have been migratory in the northeast flow across Eastern New England.  Even mesoscale models have great difficulty nailing the location of these convergent bands and that's likely to continue to be the case in the coming days, but the general agreement among guidance is to carry steady and at times heavy rain that's been offshore of the Cape onto Cape Cod late Sunday night into Monday morning, with another batch of steady and heavy rain possibly rotating onshore to the Midcoast of Maine late Monday/Monday evening.  Northeast winds will continue gusting to 40 and perhaps 45 mph on Monday based upon mixing to 925 mb, though worth noting the NMM wind gust algorithm indicates perhaps 40-42 knot gusts on Cape Cod late Monday.

Of course, this continuing northeast wind will force water up against the coastline, and as mentioned in NWS products, coastal flooding on northeast facing shorelines will continue to be a possibility through Tuesday high tide cycles with new moon on Tuesday and though the tide is not astronomically all that high midday Monday, the midnight Monday night high tide is quite high and should pose problems for many communities.

Each day on Tuesday and Wednesday, bands of rainfall will become less impressive, more breaks of sun will emerge in Northern and Western New England, but moisture will hold tough for lots of clouds on Tuesday, then on Wednesday any breaks are likely to fill back in as diurnal heating fills low level stratocu back in again.  Thursday brings a westerly wind or even a northwesterly wind and this dries us out effectively with downsloping, and warm advection keeps coming from the west, very quickly moderating the atmosphere.  Upstream highs in this airmass have been in the 80s, and we should make that jump for the end of the week.  Of course, as mentioned in the longwave pattern synopsis, shortwaves will continue traversing New England in the upper level flow, and this will breed periodic scattered thunderstorms that will be largely diurnally driven.

That's all for tonight - see you on TV in a few hours!
Matt

Sunday evening prospectus, June 14, 2009

Convective feedback will continue to be an issue this week, but to a lesser extent than the past two weeks.  This previous weekend's forecast was a tremendous challenge, but - as described in Friday morning's post (below) - *was* discernible with a great deal of effort to see through the convective issues that were affecting timing and intensity.  Did you expect this weekend's weather to be the way it was?  If you read Friday's post or saw the forecast on my Quick Forecast Page, chance are good you were prepared.  If not, you may have encountered some surprises.  Check out Friday's post to find out why weather has been tricky to predict in the Northeast the past couple of weeks.

Otherwise, the next few days will feature a gradual improvement to our weather, though the same upper level low that produced rain for much of New England the first half of Sunday will only slowly move east, meaning a cyclonic flow lingers over the region.  This cyclonic flow will spiral vorticity maximums overhead, and this will be evidenced by numerous showers and thunderstorms developing on Monday, especially as diurnal heating maximizes low level instability.  With a light easterly wind, there will be a stabilization near the seashore that will minimize convective initiation processes there, while the upper level cold pool that lingers west of the upper low, over Western and Northern New England, will help to focus heavier convection, especially where orographic lift assists.  There are two items for consideration here: 1) whether/where an easterly wind will hold a low-level marine deck in place, and 2) whether flash flooding will be a concern.  To the first point, it does look like we start with a decent coverage of low altitude gray clouds Monday AM, then find a gradual breaking up...only to fill back in with the aforementioned diurnal instability.  Fog will be found in many spots that rained Sunday, owing to ample boundary layer moisture, and especially thick in the eastern quarter of New England where the light onshore flow further saturates the atmosphere.  As to the flash flooding issue, there are signals of two to two and a half inches of rain in heavier convection from the MM5 and WRF, though the placement varies from Southwest NH to Western MA in the respective models.  I think nailing down the exact location is very difficult but the location of the cold pool and a lingering surface convergence zone at the periphery of the well-developed east wind suggests southwest or Central NH may be the zone to watch.  Flash flood guidance values are about 2.5"-3.0", so I think this is at least worth watching as isolated flash flooding is possible with weak winds aloft meaning slow moving cells.

Tuesday and Wednesday will feature a strengthening anticyclone over the coast of Maine and expanding across Eastern New England, and this will actually mean a dry air advection from the east - so one day after worrying about a marine layer, we'll probably see morning clouds linger longest in the CT River Valley on Tuesday morning and dry air presses west from the surface high center, meaning the best chance of a shower would be in Western New England, and by Wednesday, most spots should be dry and sunny.

The end of the week turns unsettled again as the longwave pattern features yet another upper level low digging across the Great Lakes and into the Northeast.  Before that happens bulk of rainfall extends from Ohio Valley to Southern Mid-Atlantic as they are squished between our anticyclone and warm, summer air across the south...with a low slowly cranking up off coast of North Carolina.  This low bears close watching, as it will be sitting over warm Gulf Stream waters and in very weak steering flow, and we very well may see close-in development of a low that may start cold core but turn warm core as it fires convection over the warm waters.  Though this system is forecasted to stay well south of New England and drift off the coast, it's something that I'll be paying attention to in the back of my mind, not only because it's an interesting hybrid feature that's fun to watch for wind/waves/rain down south, but also because a digging shortwave Thu/Fri into the Northeast means it's not entirely impossible that part or all of this energy/moisture is drawn northward.  Either way, slug of warm and moist advection Thu brings clouds and rain back.  We will be north of surface warm front, which means back to cool air.  High Thu is a bit of a crapshoot because it depends on timing of precip commencement, but based on current estimation of midday arrival, this may allow lower 60s before dropping with rain moving in.  By Friday, warm front struggles to displace cool and dense air, meaning 50s and 60s seem likely with coolest temperatures east.  The upcoming weekend will hinge heavily upon front placement, which depends on timing/intensity of low pressure winding from Eastern Lakes to New England, but right now I'm thinking convectively active Saturday, especially during the afternoon Central and South (though perhaps cooler and more showery north, where the warm front may not succeed in passing thru), then perhaps a more pronounced warm sector on Sunday with afternoon convection possible again.

That's it for now...here's to another week of fun! :)

Matt