Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
615 am EST Wednesday Nov 25 2015
An increase in the southeast gradient early this morning and warming
surface temperatures have eroded the fog which impacted much of the area last
night. S-southeast winds will undergo a steady increase during the course
of the morning /around 10 knots/ as daytime heating kicks in. Large
high pressure to the east of the region will sustain ample dry air
today...supporting mainly clear skies.
//Dtw threshold probabilities...
Previous discussion...issued 343 am EST Wednesday Nov 25 2015
Short term...today and tonight
An airmass change is now in progress across Southeast Michigan as a 925mb warm
front advances its way northward across lower Michigan. This warm air
surge is in response to a strengthening low level anticyclone across
New England and surface low pressure lifting northward across the upper
Midwest...supporting a strengthening southerly gradient across the
Ohio Valley and Great Lakes today into tonight. 925mb temperatures are
forecast to rise to +8c by afternoon. Aside from a few
cirrus...skies will remain mostly clear. Ordinarily this should
equate to highs well into the 50s. There will however be a melting
snowpack to contend with and a southeasterly surface wind trajectory
off the lakes. Nonetheless...NAM and GFS model soundings look much
too cold in the boundary layer today. Various MOS guidance appear
more reasonable...suggesting highs into the upper 40s to around 50.
Recent satellite data show a plume of high low-middle level moisture
now extending from the Southern Plains into western Iowa. This Theta-E
plume will be driven into Southeast Michigan overnight within a veering 50 knot
low level jet. The amount of moisture advecting in tonight /850mb
dewpoints rising to +6 to +7 c/ and steepening 850 to 600mb lapse
rates will overcome somewhat marginal isentropic ascent to support
increasing chances of rain...especially heading toward daybreak
Thanksgiving. The increase in clouds...strengthening wind fields and
continued warming above the shallow boundary layer all suggest a
minimal drop in nighttime temperatures off daytime highs.
Upper level low over far northern California early this
morning...with 00z radiosonde observation out of Medford Oregon indicating a 500 mb
temperature of -32 c. The bulk of this cold pool/energy still prognosticated to
reside across the Great Basin/central rockies into the weekend.
Secondary cold pool/northern stream upper level trough over Alberta
tracking through Ontario on Friday...driving cold front through
Southeast Michigan during the day. 00z NAM/GFS/Canadian all a bit
slower with the frontal push compared to last night...with the
highest probability of rainfall occurring around and just past 12z
Friday. Good moisture plume (pw values in excess of 1 inch)/850 mb
Theta-E ridge sliding into the County Warning Area Thursday night...as 850 mb dew
points reach 7 c toward 12z Friday...per 00z Euro. The daytime hours of
Thursday are a bit difficult to discern rainfall chances with the
good initial surge....but suspect at the very least the increase in
low level moisture will result in low clouds and drizzle...and thus
will lean on the higher side of probability of precipitation with few waves of isentropic
ascent rippling through. Continues to look like precipitation will
shut off late Friday before enough cold air arrives to support
changeover to snow...with good subsidence and dry air arriving
Friday night. 00z Euro/GFS both in reasonable agreement with 850 mb
temperatures settling around -2 c Saturday morning and 925 mb temperatures around
-5 c...setting US up for cool day on Saturday...with highs likely
coming up just a couple degrees short of normal values. Any low
clouds around from northerly flow off Lake Huron expected to mix out
during the afternoon hours as upper level northwest confluent flow
and surface high takes better hold into Saturday night. It appears
this high/ridge will hold during Sunday as well (noted shift in the
00z Euro compared to last night)...with just a low chance of
precipitation toward the Ohio border as low tracks through Kentucky.
Sustained southerly winds of 20 to 25 knots developing over much of
Lake Huron today. Warmer air accompanying this process will lead to
increasing Overlake stability. This will limit the gust potential
to 30 knots or less despite an increasing sustained wind. This will
result in a period of marginal small craft conditions for elevated
waves along the nearshore waters of the northern thumb region...and
have issued small craft advisories. A moderate pre-frontal southwest
flow will remain in place on Thursday. Expect a long duration rain
event late Thursday into Friday as the front slowly tracks east.
Cold frontal passage on Friday brings a wind shift to north...with
gusts between 20 and 25 knots to perhaps as high as 30 knots over
Southern Lake Huron with longer fetch into Friday night.
Lake Huron...Small Craft Advisory until 7 am EST Thursday for lhz421-441-442.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).