NWS Forecast Discussion
FXUS63 KDTX 201011
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
611 AM EDT Wed Mar 20 2019
Window of clear sky across the lowest 5000 ft this morning, as high
cloud thickens in advance of an approaching low pressure system.
Deeper plume of moisture associated within this system arrives this
afternoon and evening, leading to widespread rainfall and an
accompanying reduction of both ceiling height and visibility. Low
VFR to MVFR restrictions emerging during the late day period, with
further reduction into IFR more likely overnight as the low slides
through. Prevailing southerly winds turning modestly gusty this
afternoon, near 20 kts at times.
.DTW THRESHOLD PROBABILITIES...
* High in ceilings below 5000 ft mid afternoon through tonight.
* High in precip type being all rain.
Issued at 320 AM EDT Wed Mar 20 2019
NEAR TERM / TODAY THROUGH TONIGHT
As of 320 AM EDT...The first early morning of spring (although
technically does not start until around 6pm this evening) continues
to start off quiet across the region as surface high pressure
continues to slide eastward into the Mid-Atlantic region/New
England. Generally clear skies across the region will gradually give
way to increasing cloud cover as the morning progresses ahead of two
pieces of energy approaching from the west.
One piece of energy will be associated with a northern stream wave
diving south from the northern Great Lakes, which continues to look
fairly robust on water vapor satellite imagery. This feature will
interact with a piece of shortwave energy ejecting out of the
central Plains, with water vapor imagery also depicting a compact
circulation associated with this feature. The end result will be the
development of a weak surface low that tracks northeast across lower
Michigan later today as the upper-level PV anomalies phase.
Increasing midlevel height falls will result in a modest
low/midlevel mass adjustment as a LLJ increasing to around 40 knots
helps transport increasing moisture over the region as PWAT values
increase over 0.50 inch. Isentropic analysis depicts modest ascent
in the 1000-700 hPa layer, but bulk of dynamic lift will correspond
to arrival of height falls this afternoon/evening.
Hi-res and local probabilistic guidance are in good agreement with
precipitation holding off until after 1pm, with temperatures warming
well into the 40s and even near 50 across portions of the Detroit
metro resulting in precipitation falling as rain. Coverage looks to
be widespread, especially from 3pm-8pm when the bulk of the rain
will fall. Overall intensity will generally be light in nature, with
best coverage of more moderate rain elements focused north of the M-
59 corridor. For more on expected rainfall amounts, see the
Hydrology section below.
In addition to rain today, southwest flow will strengthen and become
gusty at times during the afternoon, with peak gusts 20-30 mph. Rain
will transition to more of a showery nature this evening as the
northern stream trough axis swings through. As the boundary layer
cools with loss of daytime heating, a few wet snowflakes will mix in
at times, especially north of the I-69 corridor. Not expecting much
in the way of accumulation with any wet snow activity outside of
maybe a dusting on some grassy surfaces. A weak cold front will move
south across the region tonight with winds becoming northwest in its
wake. Cold air advection will be minimal with the frontal passage
(850 hPa temps falling from -2 to -4 C) helping to keep low
temperatures at bay and settling in the low/mid 30s.
SHORT TERM / THURSDAY THROUGH FRIDAY NIGHT
Deepening longwave trough over the eastern seaboard will result in
north/northwest flow setting up aloft during the day Thursday.
Shortwave ridging and strong anticyclonic vorticity advection will
ensure that Thursday is dry with high temperatures in the 40s.
Trailing shortwave energy will race south through the mean longwave
trough Thursday night into early Friday morning, and will brush
eastern areas (particularly the Thumb) with scattered rain/snow
showers. 00z GFS/ECMWF are a little more aggressive with coverage,
with daytime heating during the day Friday potentially resulting in
activity lingering into the afternoon hours. The trailing wave will
usher in a brief period of colder temperatures Friday as the low-
level thermal trough swings through. Highs look to only reach the
upper 30s for most areas, with lower 40s possible closer the Ohio
LONG TERM / SATURDAY THROUGH TUESDAY
Highly amplified ridging will then set up over the Great Lakes and
Midwest for the weekend, with southeast Michigan residing on the
eastern edge of the ridging. This placement will ensure dry weather
with abundant sunshine throughout much of the weekend. Building
heights/thicknesses aloft will allow temperatures to rebound, with
highs Saturday back in the 40s, with upper 40s to lower 50s Sunday.
Ridging will break down late Sunday into Sunday night, with the
potential for precipitation along a developing cold front that will
sag south across the region. GFS continues to be more aggressive
with coverage, while the ECMWF remains drier. Given the overall
synoptic setup across North America, which looks to feature a quasi-
Omega block pattern over Canada with strong high pressure moving
from Manitoba into the northern Plains/Great Lakes, will side with
the ECMWF solution as this pattern should favor the bulk of
available moisture remaining south of the region. Aforementioned
high pressure then becomes established over the region for early/mid
next week with seasonable temperatures.
A low pressure system will settle across the region through tonight.
Moderate southwest winds will exist today locally in advance of this
system. Gusts 20 to 25 knots at times. A diminishing gradient as
the low passes through will allow winds to ease tonight. Modest
winds Thursday, before strengthening from the northwest Thursday
night. Strongest northwest winds noted Friday, when a period of
gusts to low end gales will be possible. Expect ice breakup to
continue with the changing wind conditions and relatively mild
Light rain will overspread the region this afternoon and evening.
Rainfall totals of two to four tenths of an inch are expected. Most
mainstem river levels have subsided significantly over the last
several days. The additional precipitation will only serve to slow
the steady decrease with no additional river flooding expected.
Susceptible collection areas, that likely already contain run-off
water, will swell with the additional precipitation.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
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