000 FXUS63 KGRR 291909 AFDGRR  Area Forecast Discussion National Weather
Service Grand Rapids MI 308 PM EDT Mon May 29 2023  LATEST UPDATE... Short
Term/Long Term/Marine/Fire Weather/Climate  .SHORT TERM...(This evening
through Tuesday night) Issued at 308 PM EDT Mon May 29 2023  The upper-air
pattern continues to be characterized by a Rex block over eastern North
America, comprised of an upper high over the Great Lakes an upper low over
the southeast CONUS, with fairly weak tropospheric flow over the region. At
the surface, a broad synoptic ridge is draped over the region -- extending
from a high over eastern Canada -- with a weak/diffuse surface low over the
mid Atlantic coast. This regime is maintaining weak low-level easterly flow
on a broad scale, but a mesoscale lake-breeze circulation has developed
again this afternoon, similar to recent days.  A modest increase in
southeasterly low-level flow (10-20 kts at 925 mb) tonight should
facilitate weak low-level moisture transport into the region. Short-range
guidance suggests that surface dewpoints should generally increase after
daybreak Tue as the convective boundary layer deepens and entrains this
moisture, with mid 40s-mid 50s dewpoints likely to be common across the
area during the afternoon. The arrival of this low-level moisture -- with a
corresponding reduction in convective temps (likely into the mid 80s F) --
should yield widespread cumulus development across inland areas on Tue,
given sufficient boundary-layer warming. High temps on Tue will likely be
warmer than today, reaching the upper 80s to around 90 F across inland
areas, given the expectation of warmer 850- mb temps.  12z HREF guidance
suggests that isolated showers may develop on Tue afternoon along/east of
the lake-breeze front, where cumulus may become sufficiently deep. However,
RAP forecast profiles indicate a fairly deep and dry subcloud boundary
layer, suggesting that evaporation/virga will be substantial beneath any
precip-producing clouds. Will opt to maintain PoPs below 15 percent
(threshold for slight-chance showers) with this update, but will introduce
a slight chance of sprinkles away from the lakeshore.  .LONG
TERM...(Wednesday through Monday) Issued at 308 PM EDT Mon May 29 2023  Our
focus in the long term portion of the forecast will be on rain chances,
albeit small ones, through much of the period from Wednesday through
Sunday, with some possible better chances for rain coming next Monday. We
will see a trend of warmer temperatures through next weekend, and generally
a slow increase in moisture. The moisture trend is a bit complicated, and
we will discuss further below.  Upper ridge will remain in control of the
weather across the area into early next weekend in one form or another.
This will keep widespread and appreciable rain out of the area likely into
next weekend.  There are caveats to this thinking though as we will see the
potential for a few mainly diurnal showers and storms increase slightly for
Wednesday and Thursday. What happens is that we will see a bit higher
moisture move in with a southerly component to the low level flow develop.
This will bring in sfc dew points into the 50s, and maybe even 60s by
Thursday. This moisture will combine with a fairly stout lake breeze
convergence band that is forecast by the models to pop a few showers and
storms in the afternoon. Convergence is almost maximized with a synoptic
flow from the SE, and a lake breeze from the NW developing. So while most
of the area will stay dry, there could be some locally heavy downpours with
slow moving cells as the lake breeze moves further inland.  The rain
chances might be stunted a bit for Friday and Saturday as the flow becomes
more easterly by then, and advects a pocket of much drier air over the
area. Temperatures will continue to warm at least a couple of degrees per
day with almost full sun and good mixing taking place.  We are then looking
at a pattern change poised to take place in one way or another sometime
next Sunday/Monday, with details to be ironed out. The general sense is
that some strong upper energy across Canada will drop south and push a
fairly potent backdoor cold front south through the area on the front side
of the large scale ridge across the Plains.  The interesting part of this
is that the last data shows an upper low rotating toward the area from the
NE, and a moisture feed off of the Atlantic. However this happens is quite
uncertain at this time. There is a fairly decent signal in the Ensemble
means that cooler air will replace the heat toward next weekend and then
the following week. Rain chances will be quite uncertain depending on if
the front will have some moisture to tap as it moves through, but it will
be a better chance that strong ridging overhead. 

NWS GRR Office Area Forecast Discussion