Forecast considerations

Forecast on target, and most recent update was to take care of some wording and minor timing issues.
Strong wave on water vapor continues to exit Lower Michigan this evening. This will end the light
rain around midnight for the far eastern areas. Surface front at 01z was from phn to adg. This will
leave the area in the frontal zone for much of the early night with mostly cloudy skies expected.
However a steady nw flow will continue to advect the 40s dew point air into lower Michigan and give
way to at least partial clearing by daybreak Tuesday morning. This drier air should also proclude
much fog development overnight, in addition to the lack of boundary layer moisture as indicated by
the current obs and the 00z upstream soundings.

. . posted at 923 PM EDT Monday September 15 2014

Remaining Forecast opinion - Tuesday through next Monday

Surface high pressure will be firmly centered over Iowa/Missouri Tuesday morning with a ridge
extending into Lower Michigan. Dry conditions will be ensured by this high pressure feature and
lingering mid level subsidence trailing today's system, but the cloud forecast will be less
straightforward or at least less optimistic than what might be expected with high pressure taking
control. As mentioned in the previous discussion, this past weekend demonstrated how much influence
the warm Great Lakes water can have on the cloud pattern in these early season cool air masses that
are not devoid of boundary layer moisture below the subsidence inversion. This is expected to be in
play again Tuesday with a couple of subtle differences mainly tied to model timing on the southward
placement of the low level thermal trough and broad anticyclonic low level flow. These elements add
some uncertainty to the prospects for nocturnal stratocu migrating into our area from northern lower
after sunrise. However, there is more confidence in low level cold advection combining with daytime
surface heating to fill in remaining open sky during the late morning through the afternoon.
Forecast soundings across all model runs indicate steep lapse rates below 800 mb with enough
moisture for generous coverage of shallow cumulus. The resulting partly sunny sky and low level cold
advection will limit max temps to the lower and mid 60s vs the lower half of the 70s that is normal
for mid September.

A significant diurnal component in the Tuesday afternoon cloud pattern will give way to a clearing
trend with sunset Tuesday evening. This will open the door to a cold night as surface high pressure
becomes centered over Indiana and Ohio and fosters strong radiational cooling with light wind under
mostly clear sky. Late developing westerly flow below 850 mb will be on the trailing flank of the
low level thermal trough and thus represent warm advection... Too weak and late to disrupt
radiational cooling but enough to lessen concern for stratocu from Lake Michigan. About the only
other cloud potential is associated with the next short wave embedded in the broad long wave trough
over eastern Canada. Model consensus indicates this will be too late to provide more than thin
cirrus toward sunrise Wednesday.

The cold front trailing the northern Great Lakes short wave will bring a glancing chance of light
rain to the Saginaw valley Wednesday afternoon. The front will be losing moisture integrity as it
moves south but will have a respectable temperature contrast judging from model low level thickness
and thermal fields. After a run toward 70F Wednesday afternoon this front will kick the legs out of
the warming trend by Thursday with highs back down toward the lower end of the 60s, and possibly
confined to the upper 50s in the Thumb given low level flow from the northeast.

As quickly as the surface high is reinforced through Lower Michigan Thursday, it is displaced
eastward equally fast Friday in global model solutions heading into the weekend. Substantial upper
level height falls over the Gulf of Alaska are shown to initiate progression in the long wave trough
over eastern Canada. The short wave trough helping this along is trending stronger in the latest
solutions as it moves through the plains, just in time to pick up tropical remnants of "odile" from
the four corners area. This will set the stage for a warmer but wetter weekend as the frontal system
moves through the Great Lakes.

. . posted at 348 PM EDT Monday September 15 2014


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