NWS Forecast Discussion

000
FXUS63 KDTX 312017
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
417 PM EDT Sat Oct 31 2020

.DISCUSSION...

A complex low pressure system moving through northern Ontario and
the Great Lakes remains the focal point of weather in SE Michigan
this weekend. The first result is a continuation of pleasant
conditions this afternoon into this evening for Halloween
activities. This is followed by increasing clouds and coverage of
rain showers after midnight associated with a strong cold front set
to bring in sharply colder air, snow showers, and strong wind gusts
for Sunday.

Mid afternoon observations indicate full sun getting assistance from
increasing south wind in lifting temperatures toward the lower 50s.
Gusts in the 20 mph range add a cooler feel to readings until
diminishing near sunset. Left over gradient flow in the 10 to 20 mph
range then continues feeding warmer air into lower Michigan while
adding Gulf modified continental air back into the frontal zone with
a boost from Lake Michigan. The moisture aids development of showers
as forcing ramps up considerably due to the upper level jet and mid
level trough digging strongly through the Plains producing strong
cold frontogenesis into the Great Lakes. Bands of rain showers
mature over the region within the low to mid level theta-e ridge and
ahead of the cold front in support of categorical POPs already in
the going forecast through sunrise Sunday.

The strong cold front sweeps the primary band of rain showers
eastward out of the area during early Sunday morning. The NNW wind
shift and onset of sharp cold advection could generate some 40 mph
wind gusts but more likely later as convective depth increases
through the day. The greatest potential for advisory level wind
gusts in the 50 mph range occurs mid to late afternoon coincident
with a strong reinforcement of cold air and NW gradient flow. The
increased gust potential occurs as the 500 mb trough deepens
strongly and closes off over Lake Huron which drives the low level
thermal trough into SE Michigan with boundary layer mixing access to
50 kt wind near 850 mb.

Wind gust potential also becomes augmented by convection within the
lake effect snow machine as it ramps up during the day. Model
soundings indicate this also peaks during mid to late afternoon as
convective depth approaches 700 mb after 18Z. Lake effect potential
is boosted by a background of strong and deep cyclonic flow across
the lakes tied to the closing 500 mb low combined with 850 mb
temperature dropping to about -10C over the entire lake aggregate
during the afternoon. In addition to convective depth near 700 mb,
model soundings show CAPE and supersaturation through the DGZ with
surface based melting layer sub 800 ft. These parameters support
bursts of heavier snow showers but with accumulation limited by the
tendency for cellular character during the day along with a steadily
veering wind. Upstream preconditioning by Lake Superior and Michigan
points to the Saginaw valley having the best potential for around an
inch of grassy accumulation, and also the Thumb region downstream
from Saginaw Bay that is still warm enough to supply a substantial
component of instability. Coverage and intensity is then expected to
decrease considerably Sunday evening primarily due to increasing
subsidence in confluent NW flow aloft.

The incoming upper level ridge and associated low level warm
advection develops later Sunday night through Monday. The
temperature at 850 mb warms from about -10C Sunday evening to about
+3C by Monday evening. It is a strong warm advection pattern but
lean on moisture limited to mid cloud indications rather than any
isentropic lift induced precipitation. The rapid moderating trend
sets off a pattern of dry weather dominated by a broad long wave
ridge over the continental U.S. through the mid week period. High
temperatures are projected to reach the lower 60s by Wednesday,
about 10 degrees above normal for early November.







Much quieter and warmer weather still appears on tap for next week
as heights rise and the upper level flow becomes zonal over northern
Conus/southern Canada. 850 MB temps nearing 14 C progged to arrive
by Wednesday, as upper level ridge center (586+ DAM at 500 MB) takes
residence over the southeastern United States... per 00z Euro. High
temperatures in the lower 60s appears likely Wednesday through at
least Friday with the prolong and deep southwest flow.

&&

.MARINE...

Confluent southerly flow wedged between high pressure over the
Northeast and low pressure crossing central Ontario will continue to
strain the surface gradient driving up wind speeds through the
overnight hours. Forecast trends continue to support frequent gusts
to gales over the open waters of Lake Huron, Saginaw Bay, and the
nearshore areas along The Thumb with the initial warm advection
surge as strong low-level flow crosses overhead. Upgraded the Gale
Watch to a warning for the nearshore zones while maintaining Small
Craft Advisories along the remaining nearshore zones until Sunday
morning.

The aforementioned clipper low will steer an associated cold front
through the central Great Lakes beginning Sunday morning. This cold
advection, northwest flow regime, will slam gale force winds and
gusts to gales across all marine areas Sunday into Sunday night.
Made the remaining upgrades to Gale Warnings for all zones,
effective 12Z Sunday until early Monday morning. Gusty showers are
expected ahead of the front with a change in p-type over to
rain/snow, then all snow on the back edge. Environmental conditions
within this dynamic environment and the very unstable profiles
suggest periods of snow squalls and poor visibilities. Breezy but
milder conditions begin Monday as a warming trend develops into next
weekend with drier weather and diminishing winds.

&&

.PREV DISCUSSION...
Issued at 112 PM EDT Sat Oct 31 2020

AVIATION...

Active stretch of weather through Sunday as a cold front approaches
today and sweeps through early Sunday morning. Today will be VFR
with mostly clear skies after any lingering stratocu mixed out.
Gusty southerly winds this afternoon with the tightening gradient
ahead of the front then a period of low level wind shear starting
later this evening with around 40-50 knots down to 2000ft. Narrow
ribbon of rain will accompany the cold front starting around 06-07Z
for MBS and ending for DTW around 12Z. MVFR CIGS will likely
accompany the rain and though may mix out briefly just behind the
front, but cold advection will bring widespread stratocu with
potential for afternoon MVFR or low VFR at a minimum. Rain/Snow
showers may accompany this afternoon activity but coverage may be
limited with banding features. Winds will flip to westerly in the
morning behind the front with gusts conditions through the rest of
the day.

DTW THRESHOLD PROBABILITIES...

* High in ceilings below 5000 ft late tonight through Sunday.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...Lakeshore Flood Warning from 7 AM Sunday to 4 AM EST Monday for
     MIZ049-054.

     Lakeshore Flood Advisory until 10 AM EST Sunday for MIZ063-070-076-
     083.

Lake Huron...Gale Warning until 4 AM EST Monday for LHZ361>363-421-422-441-
     462>464.

     Small Craft Advisory until 7 AM EST Sunday for LHZ442-443.

     Gale Warning from 7 AM Sunday to 4 AM EST Monday for LHZ442-443.

Lake St Clair...Small Craft Advisory until 7 AM EST Sunday for LCZ460.

     Gale Warning from 7 AM Sunday to 4 AM EST Monday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 7 AM EST Sunday for LEZ444.

     Gale Warning from 7 AM Sunday to 4 AM EST Monday for LEZ444.

&&

$$

DISCUSSION...BT
MARINE.......KK
AVIATION.....DRK


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NWS DTX Office Area Forecast Discussion