Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
628 am CDT Wednesday Oct 22 2014
Short term...(today and tonight)
issued at 327 am CDT Wednesday Oct 22 2014
Early morning water vapor imagery reveals a nice circulation
currently moving northeast into southern Saskatchewan from
northern Montana. Coupling this image with rap analyzed 500h
indicates the presence of a negatively tilted shortwave trough.
This feature has produced showers and thunderstorms overnight
across Colorado...western Nebraska...Wyoming...Montana...and
extreme western North Dakota. The trough will continue in a
general east-northeast direction through the period and will bring rainfall
to most of the County Warning Area.
This morning...hi-res guidance had been indicating an area of
thunderstorms developing in the elevated instability this morning
ahead of the main trough. The hopwrf still indicates this
potential...as do the nmm and arw. The hrrr however...dropped this
idea from its earlier runs and may be on to something given the
latest radar trends across the Dakotas. Still...kept a low pop
mention from western Minnesota and north and west of St. Cloud through
The main precipitation band will come together over the next few
hours across the central Dakotas as a continuation of the activity
seen overnight in the aforementioned areas of co/WY/NE. As we
head into the afternoon...upper level divergence thanks to an
enhanced flow at jet level to the northwest will be superimposed
with the vorticity maximum and frontogenetic forcing in the low levels
across the eastern Dakotas. Saturation will not be a problem...but
there is the potential that thunderstorms to the south side of the
expanding precipitation band could limit quantitative precipitation forecast to the north.
Hi-res models agree that the north to south oriented precipitation band
will be progressing east across western Minnesota this evening. Simulated
reflectivity from these models suggest limited returns north of
about Mankato. This lines up well with the northern extent of the
negative best lifted indice's which will be confined to areas south of about
Mankato tonight. Hence...isolated thunder remains in the forecast
for this area...while more stratiform showers are expected to the
north. In general...a range of 0.15-0.35" is now in the forecast
with the maxima being in far southern Minnesota where limited instability
may enhance rainfall rates.
Global models and hi-res models alike agreed that the precipitation band
looks to move out a bit quicker on Thursday. So...dropped the probability of precipitation
faster from west to east early tomorrow through mid-morning. Now
expecting the entire forecast area to be dry by late morning and beyond.
Otherwise...today will see strong southeasterly winds ahead of the
approaching system. Increased winds a few knots and hence the
increased highs for today saw a slight increase as well.
Long term...(thursday through tuesday)
issued at 327 am CDT Wednesday Oct 22 2014
Sharp upper level trough will shift off to the east Thursday evening while an upper
trough builds over the northern plains. The surface pressure pattern will remain
fairly baggy...generally with little definition to it. That
said...a pronounced S-SW flow will develop at the surface starting
Friday and last into the weekend. A weak surface cold front front...but more
likely a surface trough...will swing through the region Friday but be of
little consequence aside from a small increase in cloud cover
following the clearing behind the departing aforementioned trough.
The departure of this trough will spell large-scale ridging over the
central Continental U.S....spreading northward into the northern plains and upper miss
River Valley. The increases in 500 mb heights will allow for noticeable
warming for the weekend...including highs reaching and possibly
eclipsing the 70-degree mark. This warm-up looks to last for just
Saturday and Sunday...as the ridge will flatten and shift eastward late
sun into Monday in advance of a more organized storm system for the
start of next week. Though there is decent model agreement on the
formation of a storm system in the Lee of The Rockies Monday...
shifting northeastward into the Great Lakes by Tuesday...there is some model
disagreement as to the placement and speed of the system. Have
included probability of precipitation but no higher than 30-40 percent at this time. There is also
growing confidence that following this system will be a cooling of
temperatures back down to seasonable levels for the middle portion
of next week.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1053 PM CDT Tuesday Oct 21 2014
Active period ahead with rain developing to the west today and
moving east across the forecast area through tomorrow morning. MVFR likely
with IFR possible...but not indicated in the tafs at this point. Previous
tafs looked good...mainly some edits were made to the
timing of the precipitation which was heavily based on available hi-res
model guidance. Decent confidence in terms of the precipitation moving
through. Isolated thunder possible west through the moring. Sped
up the clearing from west to east late tonight through Thursday
morning. This could lead to more fog if the wind remains light
enough...mainly our west.
Kmsp...little difference from main discussion. Currently
anticipate thunder to remain west of msp this moring.
/Outlook for kmsp/
Thursday...VFR with MVFR/-shra early. Winds south-southeast at 10g20kts.
Friday...VFR. Winds south-southwest at 5-10kts.
Sat...VFR. Winds northwest at 5-10kts.