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Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI
426 am CDT Wednesday may 6 2015

Update...surface observations...calls to local law enforcement and
web cams show dense fog expanding across southwest County Warning Area. Boundary
layer winds support dense fog persisting through the early
morning. Hence posted dense fog advisory for rock...green....Lafayette
and Iowa counties. May need to expand into Dane and Sauk counties
as fog may spread northward.

&&

Previous discussion... /issued 328 am CDT Wednesday may 6 2015/

Today and tonight...forecast confidence - medium.

Showers continue to diminish across northeast County Warning Area as weak middle-level
wave over central WI progresses eastward. Small band of showers has
recently developed west of the Baraboo area...and is progressing
north-northeast. Think these showers more a result of northward moving low
level warm front as winds just off the surface...above shallow
inversion turn more south and increase. Storm Prediction Center mesoanalysis has
widespread deep layer moisture flux convergence over northern half of County Warning Area
into west central WI. Hence will continue low probability of precipitation early this morning
across north. Surge of low level warm air also contributing to fog
over southwest County Warning Area which will likely spread north toward rest of
western County Warning Area. Drier air over eastern County Warning Area likely to keep fog more
patchy this morning. Call to Green County sheriff had widespread
moderate fog over the County...but keft has been 1/4sm for an hour.
For now will issue Special Weather Statement for dense fog but may need to upgrade to dense
fog advisory through 15z or so if it continues to thicken.

Upstream short wave over northwest MO/southwestern Iowa may bring a few
thunderstorms to southern WI late this morning through the afternoon as warmer more
humid air settles over southern WI. MUCAPE expected to reach several
hundred joules this afternoon...but forcing at this time looks weak. For
now will go with low probability of precipitation and carry into the early evening. As
upstream low level jet pivots east later tonight ahead of slowly
moving plains trough...few showers or storms could skim the far west
later tonight.

Thursday - confidence...medium
middle level ridge axis proggd to the eastern Great Lakes. 500
millibar flow turns more southwest. Primary shortwave will lift
northeast through Minnesota. Much weaker ripples in the flow further east
across IA/IL/WI. Low level flow will be southerly with a warmer
and more moist regime. 925 millibar south flow sends temperatures to near
20c...so warm day taking shape with many locales likely to hit the
80 degree mark. Models continue to show potential for rain showers/thunderstorms and rain
especially during the afternoon across SC WI. This due to appch surface/850
front...weak middle level waves and diurnally driven
instability...although NAM BUFKIT soundings not overly unstable.

Friday and Friday night - confidence...medium
500 millibar flow remains southwest with surface/850 fronts
enhancing low level convergence this period. In addition...better
consensus for secondary low riding northeast along the
front...raising confidence a bit for rain showers/thunderstorms and rain into at least
Friday evening. As it stands the Gem takes the low across central WI
with the European model (ecmwf) in southern WI and the GFS across northern Illinois. In all cases
decent shot at rain showers/thunderstorms and rain so will go with likely probability of precipitation for the central
and eastern County Warning Area Friday night.

Saturday - confidence...medium
850/925 front sags south though baroclinic zone lingers for a
bit...so the European model (ecmwf) idea of keeping some rain showers around seems
reasonable...so will keep the superblend probability of precipitation going. Surface
boundary does shift south of WI though some timing differences in
this regard. Expect winds to shift NE. GFS a bit quicker than the
European model (ecmwf).

Sunday - confidence...medium
isentropic lift proggd to increase into the area. The Gem and GFS
are quicker on this with surface low lifting into western Iowa/eastern NE.
The GFS keeps the low further southwest and shows only spotty
precipitation. Will go with superblend probability of precipitation at this time.

Sunday night through Monday night - confidence...medium
500 millibar upper low proggd to move from the Central Plains
through WI this period. Surface reflection of this feature shows
varying solutions with the GFS keeping low south and a large def
zone shield of rain/thunder setting up across the County Warning Area. Meanwhile
the European model (ecmwf) has been the surface low moving through southern WI with the
Gem favoring the southern GFS solution. Despite the model
differences looks like a wet and unsettled period.

Tuesday - confidence...medium
decent cooling in the wake of the low with a drop in thicknesses
and 850 temperatures. Lingering rain showers potential within this regime.
Lingering cyclonic flow more amplified on the GFS with closed low
slower and just east of WI.

Aviation/12z tafs/...expect areas of fog this morning to thin later
this morning into the afternoon...however will likely hang on by Lake Michigan...or
else redevelop. Warmer...more humid air moving across the Cool Lake
waters will result in fog that may be dense for a time by the lake
through tonight.

Otherwise...the surge of warmer air will likely set off isolated to scattered
showers and thunderstorms later today into this evening. IFR ceilings
this morning should thin and lift inland...but may hang on by the lake
into tonight.

Marine...shifting low level winds more to the southeast will allow
warmer air with higher dewpoints to push northward across Cool Lake Michigan
today into tonight. Upstream surface observation over southwest lower Michigan into northern
Indiana showing surface dewpoints in the lower 50s. With lake surface temperatures
around 38...dense fog will likely develop over the near shore waters
today and linger into tonight. Will Post marine dense fog advisory for visibilities
of 1nm or less beginning this morning.

Increasing southwest winds in the boundary layer on Thursday should
thin any lingering fog. Southwest wind gusts will approach Small
Craft Advisory levels on Thursday as well.

&&

Mkx watches/warnings/advisories...
WI...dense fog advisory until 10 am CDT this morning for wiz062-
067>069.

Lm...dense fog advisory until 1 am CDT Thursday for lmz643>646.

&&

$$

Update...mbk

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