Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
701 PM EDT Friday Oct 24 2014
Lingering moisture from a weakening trough resulted in a strato cumulus
field which should reside over the terminals for at least the next 2
to 3 hours. There has apparently been enough daytime heating to lift
bases into MVFR for most of the area. Weak surface ridging building
in from the west is expected to scour out these clouds by late
evening. This will open the door for potential fog and or/low
stratus with subtle low level moisture advection from the west. The
current forecast thoughts remain that fog will be more probable.
However...trends with the low stratus now covering west-cntl lower Michigan
will continue to be monitored as they may slide into mbs
overnight. Another influx of low level moisture along another surface
trough will bring more widespread low clouds /MVFR or possible
IFR/ back to the region by daybreak Sat.
For dtw...given the light wind which is expected to hold through the
night and some possible high clouds...restrictions in visibility due to
fog late tonight will be kept in the MVFR category at this time.
//Dtw threshold threats...
* high confidence in ceilings below 5000 feet this evening...low
confidence overnight and medium confidence Saturday morning.
* Low confidence wind gusts will exceed crosswind thresholds
Previous discussion...issued 340 PM EDT Friday Oct 24 2014
Short term... tonight
The trend of variable cloudiness under a fragmented strato-cumulus deck
continues today. Some shortwave ridging at middle-levels has allowed
for some spotty clearing in the cloud field...and these will be the
areas most at risk for fog development tonight. Persistent surface
trough over Lake Michigan will add to the depth of a cold front
sweeping into the state tonight. Cloudiness will re-strengthen ahead
of this front tonight as warm air advection brings 850 mb temperatures up from
5 to 10c in 12 hours. Expecting a very warm night in the upper 40s
as the cold front does not quite cross into the County Warning Area by 12z. Went
warmer than guidance for mins...but could even see 2-3 degrees
warmer if overcast conditions persist all evening.
As mentioned above...there is a chance for patchy fog tonight. This
chance will be mitigated by winds around 5kt and abundant cloud
cover. Areas that clear out and radiate will hold the best chance
for developing fog...and that chance appears to be north of I-69 at
this point. Model 925mb moisture is quite robust tonight...so there
is certainly potential for cloudy/warm/fogless conditions through
12z. Depth of moisture may also add a negligible chance for
Long term...Saturday through Thursday
Saturday...southeastern Michigan will become increasingly influenced
by the anticyclonic shear zone attendent to the right exit region
of the +160 knot upper level jet maximum invading the Great Lakes
region. Dynamic tropopause maps depict this well with a sharp
gradient of potential vorticity impinging down into and becoming
established over the northern 1/3rd of Lake Huron. Low level thermal
fields and isentropic ascent forecast analyses show a favorable
structure supportive of warm air advection ascent from roughly 9-16z Saturday
morning...with warm/moist Theta-E wash. There is a good initial
surge of low level moisture capped by very strong and active
subsidence aloft. However...the moisture is noisy and there are
indications that any precipitation potential will be dependent on a
diurnal heating component. Saturation is going to be razor thin in
and around a potential thin unstable pocket between 925-900mb. Throw
in the model trying to maintain some noisy and spurious convective
shreds...and will include a chance of sprinkles Saturday morning
between 8 am and noon EDT. At minimum there should at least be some
virga echoes showing up on radar. If any precipitation were to
measure...looking at a hundredth or two maximum. Time of year requires
additional attention to clouds. There is a good trapping of low
level moisture beneath the strong inversion structure with some
hints that saturation will also exist within a few hundred meters of
the stable layer. Have gone along with persistence and the
inherited forecast to go sluggish with the exit of clouds for the
afternoon. Suspect that many areas will remain socked in clouds
until as late as 4 PM...with a rapid clearing trend thereafter.
The cold front is forecasted to clear semich between 15-21z from
north to south. Westerly winds tomorrow of 10-20 miles per hour will help the
mechanical mixing process in warming temperatures into the lower
to middle 60s...roughly 5 to 8 degrees above average.
Cold air advection occurring to the southwest of the
Ontario/Quebec upper level low will cause 850mb temperatures to
fall from 8-9c Saturday...down to 3-5c for Sunday. The upper level
ridge axis will remain well west of southeastern Michigan during
the daylight hours which gives no shot at warm air advection and
lead isentropic ascent making into the area. Not really bad though
for late October...pleasant fall conditions with temperatures
checking in right at average.
Models showing low pressure moving north through Wisconsin and
dragging a cold front through Southeast Michigan beginning Monday. Wet weather
looks likely for Tuesday...with cooler drier conditions developing
behind the front Wednesday and Thursday. Upper level feature will
bring precipitation chances back into the forecast by Friday.
Strong upper level wave of low pressure will dive into the Great
Lakes region tonight and Saturday. The dynamic system will lead to a
tight pressure gradient over the northern Great Lakes with cold air
possibly building into the northern 1/3rd of Lake Huron. It is
behind this developing frontal boundary that conditions will become
quite unstable. Uncertainty exists with regards to exactly how far
south this frontal boundary will position itself. A shift in
placement to the north would lead to relatively more subdued
conditions. Will forgo the issuance of a gale watch this forecast
issuance...but did increase forecasted windgusts solidly into the 30
knot range. Strength of the system to the northeast with established
northwesterly gradient flow will continue fresh northwesterly winds
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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