Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Green Bay WI
706 PM CDT Thursday Aug 28 2014

Updated aviation portion for 00z taf issuance

Short term...tonight and Friday
issued at 412 PM CDT Thursday Aug 28 2014

Chances for heavy rainfall and potential for flooding are the
main concerns in the short term.

Radar mosaic lapse rate showed showers and thunderstorms moving
out of the plains and decreasing in intensity and coverage as they
approached the forecast area today. Kgrb radar and surface
observations indicated some light showers in Wood County at 21z.
Models all had measurable rainfall in the area tonight as a weak
middle level short wave passes across the state ahead of a Central
Plains trough. Major differences in precipitable waters on the 12z soundings
from 0.77 inch at grb to 1.41 inches at mpx and 1.85 inches at
dvn. Easterly surface flow was keeping relatively drier air in the
area...with dew points mainly in the 50s. Moisture will reach and
then spread across the area tonight.

The short wave energy...an 850mb jet and warm front moving across
the state tonight...and some upper level divergence associated with
the right entrance region of a 300mb jet maximum will provide the lift
for showers tonight. BUFKIT time/height sections showed little
instability overnight tonight...so downplayed thunder and went
mainly with showers through 12z. The 12z NAM and European model (ecmwf) had an axis
of higher quantitative precipitation forecast tonight that extended across north central Wisconsin
while the 12z GFS had it more to the northwest and outside the
forecast area.

Have showers and thunderstorms going on Friday...though there
could be a lull in the activity late morning/early afternoon as
some relatively drier air works its way into southern Wisconsin.
The upper jet support looks better on Friday as the middle level
trough approaches and BUFKIT was showing convective available potential energy in excess of
1000j/kg in central and east central Wisconsin. Storm Prediction Center had a see
text...5 percent chance for severe...for most of the area on day 2
and this seems reasonable. The NAM and ec showed the axis of
highest quantitative precipitation forecast across northern Wisconsin while the GFS was farther
north...in far northern Wisconsin and Upper Michigan.

Though heavy rainfall is likely at times...rainfall totals
through 00z Saturday should not pose much of a flooding risk but
could set the stage for flooding in subsequent periods if more
heavy rain falls.

Long term...Friday night through Thursday
issued at 412 PM CDT Thursday Aug 28 2014

Heavy rainfall potential for Friday night and Saturday...and precipitation
trends with a frontal passage Sunday night and Monday...will be
the main forecast concerns.

Have detailed the precipitation trends...heavy rainfall threat and flooding
potential for Friday night and Saturday in the hydrology section.
Any lingering showers and storms should pull out of far NE WI
early Saturday evening...with dry conditions expected for the
rest of Saturday night and Sunday.

A short wave trough and associated cold front will move through the region
late Sunday night into Monday...and produce another swath of
significant precipitation. Depending on the timing of the front...and
amount of cloud cover and heating that occurs ahead of it...severe
thunderstorms may be a concern on Monday. Increasing deep layer shear and
middle-level winds will be favorable...but instability is in
question.

The rest of the work week does not look as active...with a small
chance of showers expected on Tuesday as a short wave trough moves through...
and a chance of rain showers/thunderstorms and rain toward Thursday as a warm front
approaches,.
&&

Aviation...for 00z taf issuance
issued at 704 PM CDT Thursday Aug 28 2014

Ceilings will gradually lower to MVFR from southwest to northeast
through 06z Friday as more solid area of light to moderate rain
over southeast Minnesota to near lse moves east northeast. As low
level jet increases toward midnight...precipitation area will
continue to expand and increase in intensity resulting in more
widespread moderate rain and IFR ceilings especially at central
Wisconsin taf sites after 06z. Band of moderate rain will continue
to push northeast into northern Wisconsin after 12z Friday.
Continued influx of moisture and increasing elevated instability
early Friday may also help to generate a few embedded
thunderstorms toward daybreak Friday...but confidence not very
high. Latest model guidance suggest this initial band of
precipitation will push toward the Wisconsin-Michigan border by
around 18z...with more scattered shower activity in its wake. More
shower and thunderstorm activity is then expected to redevelop
over much of the forecast area Friday afternoon...amount of
thunderstorm activity will depend somewhat on amount sunshine that
can be realized during the afternoon. Regardless...expect
generally MVFR ceilings with areas of IFR ceilings in heavier
precipitation.
&&

Hydrology...
issued at 412 PM CDT Thursday Aug 28 2014

Latest forecast models continue to show several rounds of rain and
thunderstorms tonight through Saturday. The first round will accompany an
800 mb warm front and 30-40 knots low level jet tonight into early Friday...along
with the return of a very moist air mass (pwats approaching 2
inches). Expect some areas to get 1-2 inches of rain with this
episode. A decrease in the rain is expected Friday morning...but
additional convection should redevelop Friday afternoon...and may
become enhanced over the northwest half of the County Warning Area on Friday night as an
upper trough and surface low approach from the west...and a coupled
upper jet pulls through. The threat of heavy rainfall should
slowly shift southeast on Saturday...in association with the movement of
the upper trough and surface low. Additional rainfall amounts of 1-2
inches appear likely Friday night into Saturday...with local
higher amounts.

Will continue with the esf this afternoon. Will need to monitor
how tonight's event unfolds...then determine if any of the
subsequent heavy rainfall will overlap areas that get hit tonight.
Think the main flooding threat will be Friday night...and mainly
in urban areas and on smaller streams that are more prone to flash
flooding.
&&

Grb watches/warnings/advisories...
none.
&&

$$

Short term.....Mg
long term......kieckbusch
aviation.......esb
hydrology......kieckbusch

National Weather Service Glossary of Abbreviations