Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1107 PM CDT Tuesday Aug 19 2014
Short term...(this evening through wednesday)
issued at 254 PM CDT Tuesday Aug 19 2014
Afternoon water vapor imagery shows the healthy shortwave trough
over Lake Superior. This is the feature responsible for the stratocu
clouds and spotty showers across much of the area today. This shower
activity will diminish this evening with the loss of daytime
heating...and skies will gradually clear from west to east as the
aforementioned trough shift farther east. The concern then becomes
the potential for fog development...as the surface ridge settles in
and winds decouple. The highest potential for dense fog appears to
be along the Minnesota/WI border toward Eau Claire...in closest proximity
to the surface ridge axis...South/West of the area in northern WI
where clouds may linger into early Wednesday morning. Have included
a mention of 1/2sm in the grids/forecast...but will need to monitor
trends closely this evening to see if dense fog will become more than
just patchy in nature and necessitate an advisory.
Areas of fog will burn off by middle morning Wednesday...and
temperatures will moderate into the upper 70s to middle 80s range.
Clouds will move in from the west during the afternoon as a
shortwave trough tops the middle level ridge and approaches the area.
Could see some showers and thunderstorms sneak into west central Minnesota
during the afternoon hours as 850-700mb temperature/moisture advection
Long term...(wednesday night through tuesday)
issued at 254 PM CDT Tuesday Aug 19 2014
The long term continued to show two main rounds of convection. The
first developing late Wednesday into Thursday...and again Friday
night into Saturday. Overall the the previous forecast was in good
agreement with the models...so did little in the way of significant
Models indicate short wave moving across the Dakotas Wednesday
with this developing a warm front which will lift northeast into
southern Minnesota late Wednesday night. Strong 850 mb moisture advection
develops out ahead of the front and should lead to MS development
mainly after 06z Thursday into southwest/west central Minnesota and then roll
east into east central Minnesota by 12z Thursday. With precipitable waters
topping 2 inches with the short wave...we will likely see some
widespread heavy rains in excess of 1 inch over much of central into
east central Minnesota into Thursday morning. The main mesoscale convective system should work
east and weaken Thursday morning...leaving a fair amount of
cloudiness...especially over the northeast County Warning Area. This will limit
temperatures a bit...and did decrease them a bit because of the
uncertainty of cloud cover. The warm front should either move into
southeast Minnesota Thursday or into Thursday evening as the short wave
clears the area. Continued to taper probability of precipitation some into Thursday night
with middle level cap building northeast of the surface warm front.
Models diverge some in timing of the next short wave that lifts
northeast into the area. The GFS retains the blocky ridge to the
east over the Great Lakes and rides the wave northeast across the
area Friday night into Saturday. The 12z ecwmf is slower...lifting
most of the energy affecting our area Saturday into Sunday. Will
have to maintain some chance probability of precipitation during this period because of
model uncertainty of handling the western Continental U.S. Trough and
surface front. The 12z ecwmf does take the front through the area by
Sunday evening...with drier and cooler conditions developing that
should linger through next Tuesday.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1107 PM CDT Tuesday Aug 19 2014
Winds have become calm over all of Minnesota except the far western part.
Still some very light north-northeast wind in west central wisc plus some scattered-
bkn080. The scattered-broken clouds will continue rotating through western
wisc and fog should therefore hold off at krnh/Keau for a little
while...then go IFR or LIFR after 08z-09z. Good chance for LIFR
fog in central and east central Minnesota as well where there was spotty
precipitation Tuesday. Will not be surprised if kstc to Keau experiences
some 1/4sm fog for a couple hours around and before sunrise. Fog
should dissipate after 14z. Surface high should be in far western
Wisconsin at 12z. Tightening gradient should keep southeast winds around
10 kts in Minnesota through Wednesday evening. Surge of precipitation should
arrive in western Minnesota Wednesday evening with thunderstorms a decent bet.
fog is anticipated...and should certainly be around 4-5sm...but
confidence is not great that visibility will drop below 3sm
Wednesday morning. In part because of current temperature/dpt spreads and
light north-northeast wind. Satellite airports will certainly have some dense
fog. Surface high departs wedneday morning and southeast wind will remain
nearly constant through the afternoon. Some cumulus should develop
around midday...then storms may move in late Wednesday night.
/Outlook for kmsp/
Thursday...VFR. Chance MVFR with thunderstorms and rain...especially in the morning.
Winds southeast 5-10 kts becmg SW.
Friday...VFR. Slight chance MVFR with thunderstorms and rain. Winds NE 5-10 kts.
Sat...VFR. Chance MVFR with thunderstorms and rain. Winds SW 5-10 kts becoming northwest.