Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
726 am EDT Thursday Jul 30 2015
A mostly clear sky through the taf period with dry conditions in
place. Westerly winds will increase today south of strong low
pressure over Canada. This in combination with daytime heating will
lead to afternoon gusts into the 20 to 25 knot range. The gustiness
will cease with the loss of heating through the late evening period.
//Dtw threshold probabilities...
* low in westerly wind gusts exceeding crosswind thresholds this
Previous discussion...issued 356 am EDT Thursday Jul 30 2015
Short term...today and tonight
Southeast Michigan easing into the first of several days dominated
by a progressive lower amplitude northwest flow...Great Lakes
positioned along the southern periphery of lower heights associated
with the closed low anchoring over Hudson Bay. This flow lacking in
any meaningful middle level energy or moisture advection over the next
24 hours...leaving an environment characterized by lower humidity
/dewpoints in the 50s/ and plenty of clear sky. Deep mixing
potential today as daytime heating under full insolation and a
cooler 800-900 mb thermal layer establishes a steep low lapse rate
environment by afternoon. The contribution of this process to the
existing airmass continues to support expectations for highs to
reach the middle 80s. This will also provide a gust component
/25-30 miles per hour/ to the existing firm westerly gradient for the latter
half of the day.
Gusts will cease with the loss of heating through the evening
period. A modest west wind still left behind with the region
positioned between the deep low across southern Canada and high
pressure over the plains. This will limit the magnitude of
nocturnal cooling...environment otherwise favorable for stronger
radiationl cooling given the deep column drying and clear sky. Lows
generally lower 60s...with some upper 50s likely in the coldest
Long term...Friday through Wednesday
The longwave pattern continues to shift toward having a broad trough
set up over the Great Lakes and eastern Seaboard. This will bring
lower temperatures...intermittent cloud cover...and several
opportunities for precipitation. Guidance is supporting highs only
in the 70s by early next week.
The strong shortwave lifting through Ontario today will take up
residency over Hudson Bay throughout the next week. Nearly zonal
flow across the northern tier of the Continental U.S. Will slowly become nwerly
as the trough deepens and the subtropical ridge amplifies over the
West Coast. The resultant northwest flow into the Great Lakes will usher in
a series of waves this weekend and into next week. Early on with the
trough still developing...the energy/waves will struggle to make it
too far into lower Michigan presenting only a slight chance of any showers
developing. Best chance will be over middle Michigan and The Thumb closer to
the feature as good low/middle level lapse rates develop with some cape
aloft due to cooling in the middle/upper levels. Having said that...dry
air throughout the column will make it hard for much to hit the
ground. Another day of deep bl mixing will present itself Friday as
well...with mixing depths making a run at 700 mb. This will likely
tap into winds as high at 30 knots at that pressure which will lead
to another gusty afternoon over Southeast Michigan.
Saturday and Sunday will hang onto somewhat warmer temperatures as
westerly flow persists across the region providing weak warm air advection. Models
have been advertising several weak shortwaves tracking over and
nearby southern Michigan through the weekend but they are currently too
weak in a progressive pattern to be able to add much definition to
the forecast. A somewhat stronger upper level trough will rotate
around the parent low Sunday night into Monday. Chance probability of precipitation are
warranted for that period which has the best potential for activity
in the extended.
Post frontal west-northwest winds will increase today as the pressure gradient
tightens and deep mixing commences. Winds will gust to around 25
knots over land which will likely carry into the nearshore zones to
some degree. Small craft advisories are in effect for all nearshore
zones for today to account for the increased offshore flow. A
similar setup may necessitate additional advisories during the day
Lake Huron...Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this
evening for lhz442-443.
Small Craft Advisory from 11 am this morning to 8 PM EDT this
evening for lhz421-422-441.
Lake St Clair...Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this
evening for lcz460.
Michigan waters of Lake Erie...Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this
evening for lez444.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).