Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
704 am EDT Fri Oct 28 2016
Moisture near the surface and strong radiating have combined with
widespread cirrus to create chaotic and highly variable vsbys this
morning. Most locations have bounced around between VFR and IFR with
a few LIFR mixed in. On average, expect a trend toward improving
vsbys even before sunrise as high clouds take their toll on existing
low vsbys. All sites should be VFR within a couple hours of sunrise.
South wind today will be accompanied by VFR with just a low chance
for an MVFR ceiling around midday as morning moisture mixes out.
Confidence too low to include in the forecast at this time. Strong low-level
jet will work through the area tonight creating wind shear issues at
all Southeast Michigan airfields. Increasing concern for the low level jet inversion
to be accompanied by expanding MVFR by early Saturday morning.
//Dtw threshold probabilities...
* low for cig aob 5kft around midday. Medium after 10z Sat morning.
issued at 409 am EDT Fri Oct 28 2016
Calm winds, clear skies and moist antecedent ground conditions has
led to the development of fog this morning. Thus far, behavior and
tendencies offered by the surface observations suggests that any
dense fog is not widespread. Infrared imagery supports that a considerable
amount of high cloud will be overspreading southeastern Michigan
prior to daybreak. With the potential for high cloud to disrupt the
favorable environment for fog development, will address the fog by
including areas of in the morning forecast and handle additional
messaging through special weather statements.
The low level flow will remain decidedly anticyclonic today with
southeasterly 950-850 mb trajectories holding firm right through the
early evening hours. This will be in response to a center of the
surface ridge axis building into portions of Northern Ohio and the
Central Lake Erie Basin. Increased insolation alone will allow
temperatures to moderate back to late October averages with highs
today reaching the lower to middle 50s. An accurate message for
skies today is likely partly cloudy.
Low level warm advection will ramp up considerably tonight,
predominately occurring above the base of the subsidence inversion
that will be down to 950mb. Strong low level jet forcing will likely
keep conditions mixed at the surface with southwesterly winds locked
in the 15 to 20 knot range overnight.
The features of interest for the weather Saturday are a cold front
that will be slipping southward from northern lower Michigan and a
secondary higher Theta E gradient that will become organized west to
east across southern sections of the cwa. There is large scale
ridging that is forecasted to hold over the state which will make it
very difficult for precipitation generation through the daylight
hours. Perhaps there will be an isolated light shower possible
across the southern cwa after 21z as there is some support for the
850mb cold front convergence to accelerate southward and accentuate
some moisture dewpoint gradient aligning over the south.
The item that still needs attention is what sort of surface winds
will be possible Saturday with increased gradient flow in advance of
the front. Specifically, with low level jet overhead what kind of
gust potential will exist during the late morning as planetary boundary layer deepening
taps into the higher momentum. Right now the forecast is in the 20
to 25 mph range.
The midlevel cold front will become a stationary baroclinic zone
directly over southeastern Michigan early Saturday evening as very
brief upper level jet equilibrium period develops. This will change
during the latter half of Saturday night as enhanced region of upper
level divergence from right entrance region passes overhead of southeastern
Michigan. There is some discussion about a lack of continuity in the
nwp with regards to the upper level dynamics. However, looks like a
prototypical event for a regional stratiform precipitation event.
Given the wavelength of ridge trough couplet and magnitude of
temperature gradient the setup appears favorable for solid 850-700mb
fgen that the NAM does zero in on. Predictability appears high
enough for categorical mention of rain for late period 2. What is of
lower confidence is what sort of quantitative precipitation forecast potential exists. The event
will have the potential for the development of mesoscale fgen
banding. With forecasted precipitable waters of up to 1.40 inches, the potential
for heavy rainfall would exist under such scenario. Something to
watch for in future hi-res solutions. End timing for the event is
also in question but should be done by 15z Sunday morning.
Strong southerly winds will develop this afternoon as low pressure
tracks north of Lake Superior. A gale watch remains in effect for
the open waters of Northern Lake Huron. Sustained near-gales will be
possible at times...but stable southerly flow will limit wind gust
potential. The watch will therefore not be upgraded to a warning at
this time. Gusty fresh northwest winds will build across Northern
Lake Huron on Saturday followed by more tranquil conditions into
early next week as high pressure dominates.
Lake Huron...Small Craft Advisory from 6 PM this evening to 10 am EDT Saturday
Gale watch from 6 PM EDT this evening through late tonight for
Lake St Clair...Small Craft Advisory from 6 PM this evening to 10 am EDT Saturday
Michigan waters of Lake Erie...Small Craft Advisory from 6 PM this evening to 10 am EDT Saturday
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