Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1151 PM EST sun Nov 23 2014
A solid shield of light to moderate rain has made its way over the
taf sites as a warm front continues to lift north from the southern
Great Lakes. The rain has lowered conditions to IFR/LIFR across all
locations. Gusty southeast winds have kept visibility restrictions in check.
Winds will turn SW Monday morning in the warm sector...with wind
gusts of 30 knots expected later Monday morning. Gusts will then
reach or exceed 40 knots Monday afternoon as a strong cold front
sweeps through the area.
//Dtw threshold threats...
* high confidence in ceiling below 5000 feet tonight through
* High confidence in rain as precipitation type through Monday.
Previous discussion...issued 416 PM EST sun Nov 23 2014
Short term... late afternoon through tonight
Middle afternoon satellite imagery indicates an energetic upper air
structure over the Mississippi Valley from the Midwest to the Gulf
Coast that will drive a widespread precipitation pattern over Southeast
Michigan tonight. A smaller wave entering the western Great Lakes
will complement the stronger wave moving north from the central Gulf
Coast to form a broad area of surface low pressure over Illinois
during the evening. Middle afternoon radar composite already shows rain
filling in between the upper waves from Illinois through Kentucky to
the middle Atlantic coast. Moisture transport accelerated by the low
pressure development near the south end of Lake Michigan will only
enhance the rain pattern as the system moves through the Great Lakes
region in support of the going categorical forecast. Model quantitative precipitation forecast near
1 inch looks reasonable for the night based on the organization of
the surface warm front across solutions which will support a low
level jet near 60 knots. The result will be a high end rainfall
event for this time of year but not excessive...especially
considering the low and warm front will be progressive through the
region. There is good model agreement placing the warm front over
Northern Lake Huron leaving Southeast Michigan in the warm sector of the
surface low by 3 am. This will help lift temperatures into the lower
50s and break up rain shower coverage by sunrise Monday.
Long term...Monday through next Sunday
Several features catching attention on water vapor imagery this
afternoon...the upper wave lifting up towards the area from the
Mississippi River valley...shortwave energy diving through New
Mexico...and more upper energy over Montana/Wyoming. Models seem to
have a good handle on the latter two features...lending confidence
to how the model solutions handle this energy despite it being a
slight change from the previous model suite. New Mexico shortwave
looks to ride through the longwave trough and into the southern
Great Lakes early tomorrow while the trough over Montana/Wyoming
looks to reach the western Great Lakes tomorrow afternoon/evening.
These upper features will work in conjunction to extend surface
pressure falls back through lower Michigan....while the main surface
low lifts from northern Michigan into northeast Ontario. This has
led to a southward shift in where the stronger core of winds may be
located Monday afternoon and evening...and also necessitated a
ramping up of snow accumulations over the Saginaw Valley and thumb
Monday night. There is still some time for models to adjust tracks
and surface features further...and both elements will need to be
watched closely. Changes are still quite possible.
Main concern will be wind potential Monday afternoon into Monday
night. 12z model suite was consistent with bringing a very strong
cold front through the area during the afternoon. Model forecast
soundings show strong cold air advection occurring within a 3 hour
period behind the front...with a rapid steepening of low-level lapse
rates and deepening of the boundary layer. This instability should
be sufficient to allow a strong core of 40-50 knot winds to descend
to the surface as gusts....with the support of a tight surface
pressure gradient and strong descent along isentropic surfaces
between 280-290k. Have issued a high wind watch for the entire area
with the expectation of reaching at least Wind Advisory
criteria...although some uncertainty exists on the north end of the
watch given recent model shifts. Cold air will also allow rain to
change over to snow in the evening. Latest round of models show the
trowal working through central Michigan and Lake Huron during the
evening...and with the additional surface pressure falls extending
further south now...deformation axis has been drawn southward in 12z
model runs. This looks reasonable and did bump snow accumulations up
to 2 to 3 inches for Monday night across the Saginaw Valley and
thumb...with best accumulations expected over Midland and Bay
For Tuesday...mid-level deformation axis should provide some
moisture within the dgz...in addition to lake effect processes
kicking in with steep low-level lapse rates and deep saturation up
through 7000 feet over Lake Michigan. Moderate west-northwest flow
will favor areas between I-69 and I-94. H850 temperatures back down to -8
to -10 c will support maximum temperatures only in the middle 30s.
High pressure will build into the Ohio Valley and southern Great
Lakes late Tuesday and Tuesday night. This will provide drier air
aloft (including within the dgz) and lowering inversion heights over
Lake Michigan. Though lake effect snow should wind down...continued
westerly flow off the lake and low inversion heights should keep
stratus around overnight...with potentially some breaks near the
500 mb trough remains in place...and lower atmospheric reflection
also supports temperatures running at least 10 degrees below normal
in the extended period. A few clipper systems look to bring
measurable snow to the area. Even if these clippers miss Southeast
Michigan...expect lake response and low level convergence to support snow
showers reaching the County Warning Area. While 850 mb temperatures hover around -10c through
Friday, the European model (ecmwf) points to Saturday beginning a potential warmup.
Low pressure will lift into the Great Lakes tonight...bringing an
increase in south to southeast winds across the central Great Lakes.
The persistent feed of easterly winds will allow waves over the Lake
Huron nearshore waters to build between 3 and 5 feet tonight. A
Small Craft Advisory is now in effect.
The low pressure system will lift from Michigan to Ontario on
Monday...pulling a powerful cold front through the central Great
Lakes during the afternoon and evening. The combination of a tight
surface pressure gradient and instability generated as much colder
air rushes back in over the warmer lake waters will allow a strong
core of gale force...or even storm force...winds to develop over the
central and southern Great Lakes. Some uncertainty remains with
where this core of winds will be located...but the latest round of
forecast guidance does support a southward shift in the strong wind
core. The storm watch has therefore been extended southward to cover
most of Lake Huron in addition to Lake St Clair and Western Lake
Erie. Potential exists there for wind gusts to reach 45 to 50 knots
Monday into Monday night. Winds should begin to decrease early on
Tuesday as the low lifts away from the area and high pressure builds
into the Ohio Valley by late afternoon.
Those with interests in the Great Lakes Monday and Monday night
should follow changes in the forecast closely and watch for possible
upgrades to gale or storm warnings.
Michigan...high wind watch from 10 am EST Monday through Monday evening for
Lake Huron...storm watch from 10 am EST Monday through Tuesday morning for
Small Craft Advisory until 10 am EST Monday for lhz421-441>443.
Gale watch from Monday afternoon through Tuesday morning for lhz361.
Lake St Clair...storm watch from 10 am EST Monday through Tuesday morning for
Michigan waters of Lake Erie...storm watch from 10 am EST Monday through Tuesday morning for
Low water advisory from 3 PM Monday to 4 PM EST Tuesday for lez444.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).