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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
703 PM EDT Thursday Jul 30 2015



Aviation...

VFR conditions will continue through taf period. Strong gusty
westerly flow will subside around 00z with loss of diurnal mixing
only to resume late Friday morning. Dry airmass will lead to
diurnal few-scattered cumulus at or above 5k feet.

//Dtw threshold probabilities...

* low in westerly wind gusts exceeding crosswind thresholds this
afternoon and Friday.

&&

Previous discussion...issued 344 PM EDT Thursday Jul 30 2015

Short term...tonight

Large high pressure and upper ridge over the western half of the
U.S. And a strong stack low over Hudson Bay is providing a strong
northwest flow over the Great Lakes along with ample dry air. Gusty
westerly surface winds and a few diurnal cumulus will dissipate this
evening with lost of heating and mixing. Clear skies tonight
with dew points mostly in the 50s would allow a nice drop in
temperatures overnight. However...there is a tight enough surface
gradient to keep up westerly winds which will prevent temperatures from
radiating all the way down to dew points. Expect upper 50s to low
60s.

Long term...Friday through next Thursday

Upper low now spinning over Hudson Bay will remain there through the
weekend. Several small shortwaves are forecast by model guidance to
pivot around the system and across the Great Lakes during this
timeframe. The wave on water vapor over central Saskatchewan/Manitoba
will be the next one to affect US...and is expected to track across
northern Michigan tomorrow afternoon. This wave is not expected to
arrive with much moisture...and should only be accompanied by a weak
cold front/wind shift with the surface trough. Still...low and middle
level lapse rates are expected to steepen as the surface heats and
cooler air aloft arrives. This may support some isolated
showers/tstorms...mainly north of the I-69 corridor...which will be
closer to cooler air aloft...slightly better moisture...and forcing
from the surface trough. Boundary layer will remain well-mixed
tomorrow...again supporting westerly gusts to around 30 miles per hour and
maintaining high temperatures well into the 80s. Subsidence behind
the upper wave should then provide dry weather for at least the
first half of Friday night...before the next shortwave arrives late
Friday night into early Saturday.

This wave looks to dive slightly further south and provide a little
bit more in the way of moisture per latest model guidance as it
tracks through the central Great Lakes. Again...steepening low level
lapse rates...moisture and forcing from the surface trough and weak
cold front settling into the area will provide a chance for
showers/tstorms. Coverage once again looks limited by
moisture...slightly warmer middle-level lapse rates than the day
before...and the possibility early day timing of forcing will not
line up with peak heating. Kept a broad coverage of probability of precipitation in the
forecast for Saturday for this forecast cycle as timing these
smaller waves from the northwest is usually tricky...but probability of precipitation may
need trimming for the latter part of the day. Activity at this time
looks to remain north of the city of Detroit.

Additional shortwave energy looks to pivot around the Hudson Bay low
Saturday night and Sunday...with smaller waves tracking through the
northern Great Lakes and a larger shortwave dropping from the
northern plains down through the corn belt and Ohio Valley. This
larger wave may provide a better trigger for precipitation again
late Saturday night and Sunday...this time focused more over the
southern portion of the forecast area as steep middle-level lapse rates
are maintained by cool cyclonic flow aloft around the upper low.
Models have been showing a lot of inconsistencies with this
wave...and will likely need to fine-tune probability of precipitation as we get closer. With
the lack of a good surge of low-level cold air...maximum temperatures should
remain in the 80s.

Impressive closed low at 500 mb will allow for large scale ascent on
Monday increasing probability of precipitation throughout the day. Uncertainties in timing
of shortwaves through the southern edge of the large scale trough
will keep probability of precipitation below 60. As the 500 mb low slowly progresses
eastward...N/NW flow will setup for the remainder of the extended
forecast keeping temperatures and moisture unseasonably low. Next
chance for probability of precipitation exists in the southern tier of the County Warning Area Wednesday
night into Thursday morning...although probability of precipitation will be quite low due to
strong model disagreement.

Marine...

A well-mixed boundary layer behind a cold front will continue to
allow westerly winds to gust into the 18 to 21 knot range through
the early evening. With the expectation for winds to remain a little
lighter than expected...the small advisories have been cancelled.

An upper level disturbance passing through the area tomorrow...with
another approaching on Saturday. This will provide gusty west winds
again each afternoon. Speeds are expected to remain just under Small
Craft Advisory criteria at this time...but this will be need to be
continuously reassessed.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
&&

$$

Aviation.....SC
short term...drc
long term....hlo/ps
marine.......hlo



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