Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
1139 am CST Thursday Nov 27 2014

Short term...(today through friday)
issued at 322 am CST Thursday Nov 27 2014

Clear skies...fresh snow...and building high pressure resulted in
another very cold morning across the region. Overnight lows are
expected to be below zero across most of Minnesota...with
lingering clouds keeping Wisconsin slightly warmer. Today the cold
will continue as highs struggle to reach double digits. Later
tonight light snow will develop out west and spread eastward along
and north of i94 Friday morning which will slicken up the roads
for the black Friday shoppers.

The aforementioned surface high pressure will gradually shift
southeast today. The 27.00z radiosonde observation at kmpx measured -10c at
h850...which is quite mild compared to the -22c recorded just
upstream at kinl. As the cold air to the north advects across the
region it will limit the afternoon heating resulting in another
day where the high temperature will be colder than the normal low
temperature.

Later tonight a weak shortwave resulting more from the speed shear
along the poleward side of the jet than from actual curvature
will cause a warm front to lift through the region. Isentropic
ascent along this frontal boundary will lead to a band of light to
moderate snow that will be heaviest along and north of the I-94
corridor. Forecast soundings show a favorable veering wind
profile...but the Omega is fairly weak throughout. This is because
there isnt a deep frontogenetic circulation which would focus the
ascent along a narrow corridor. There is still an fgen
signal...especially in the h850-700mb layer...but it is a
transient feature that doesnt become anchored and allow for
persistent banded precipitation and high snowfall totals. Feel the hires
arw and nmm models have a good handle on the northwest/southeast
oriented area of precipitation that develops across western Minnesota late
Thursday evening...and slides eastward over western WI Friday
morning.

Sref plumes show snowfall totals will be only around 1-3
inches...with a 4 inch total at the high end of the spread in the
northern counties of the forecast area. This seems reasonable and
by itself wouldnt normally have much impact. However...it will
be most intense early Friday morning when shoppers are driving to
the stores...so issued a Winter Weather Advisory northeast of I-94
where the heaviest snow is expected. The airforce snow ratio
method of the GFS had 20:1 ratios...which seem high considering
the weak lift and narrow dendritic growth zone...but the veering
wind profile and relatively weak surface winds should support
ratios that are higher than climatology in the advisory area.
Finally...forecast soundings show the potential for freezing
drizzle/wintry mix on the back side of this area of snow once the
ice crystals disappear. This could lead to a light glazing if it
does indeed occur.

Long term...(friday night through wednesday)
issued at 322 am CST Thursday Nov 27 2014

Once the low pressure system over the north-central Continental U.S. That is to bring
additional wintry precipitation to the weather forecast office mpx County warning forecast area exits the area Friday
night...there is very little to speak of for additional
precipitation through the rest of the extended portion of the
forecast. That said...mid-winter type cold air will be highly
prevalent through the end of this week into the first half of next
week.

The low pressure center over the Dakotas at the start of the period
will shift east-northeast into southeast Canada while its trailing cold front slogs
through the northern plains and upper miss River Valley. Some residual
warm air aloft atop the influx of colder surface air with just enough
pre-frontal lift and moisture will squeeze out some lingering
areas of -fzdz and/or -shsn over far northern and eastern portions of the
County warning forecast area Friday night. Drier/colder air rushing in on the backside will
efficiently keep this icy precipitation to a minimum...but still may
produce some glazing over any snow received earlier on Friday. The
other concern will be fog developing in southern portions of the County warning forecast area
Friday night into Sat morning with the warmer air over the colder
recently-received snow...so have indicated as such with fzfg in
gridded and text products. Clouds will linger Sat into sun as the
frontal system in its entirety will be slow to exit the
region...but am not expecting any further precipitation as moisture will
be lacking and best lift will be east of the County warning forecast area. High pressure then
takes hold Sun night through Monday night...resulting in clearing
skies but much colder conds. The roller coaster trend in temperatures
will continue going into the middle portion of the week as
indications are that another weak low pressure system driving eastward across
the northern tier states brings warmer southerly air but also possibly
additional wintry precipitation. However...this system is rather weak and
there is considerable model disagreement so have kept slight chance
probability of precipitation confined to only far southern portions of the County warning forecast area.

Regarding the temperature trends...will look for the warmest day of the
next 7 days to occur on Saturday as highs hit the middle 30s to near
40. But...as mentioned above...the cold frontal passage looks occur by Sat
night so this brings a return of teens and lower 20s for highs on
sun into Monday. Temperatures do looks to increases on Tuesday and Wednesday but still
remain below the freezing mark.

&&

Aviation...(for the 18z tafs through 18z Friday afternoon)
issued at 1130 am CST Thursday Nov 27 2014

Trend of previous forecast looks good. Maybe an hour earlier in
snow developing. The current VFR conditions will give way to
MVFR/IFR snow over the Minnesota sites through 06z...and shortly
thereafter into WI. The freezing drizzle threat remains low for
widespread activity at the end of the snow event. Confidence
remains low as well. Will continue to mention at kmsp for continuity
sake. The better chance may move in late Friday afternoon/evening
as the surface warm front and possible lower stratus affects the area.

Kmsp...
confidence is high that light snow will spread across kmsp late
tonight into Friday morning. The most intense rates should be
less than a half inch per hour as the system moves through. Once the
main band of snow moves east forecast soundings still show some
potential for freezing drizzle...so mentioned this starting after
11z on Friday. The best chance for this may move in again Friday
evening with lower stratus and some fog as the warm front lifts
north.

/Outlook for kmsp/

Sat...VFR. Winds S at 5-10 kts.
Sun...VFR. Winds northwest at 15g20 kts.
Monday...VFR. Winds northwest at 5-10kts...becoming S late.

&&

Mpx watches/warnings/advisories...
Minnesota...Winter Weather Advisory from 10 PM this evening to 4 PM CST
Friday for mnz042>045-050-052-053.

WI...Winter Weather Advisory from 10 PM this evening to 4 PM CST
Friday for wiz014-015.

&&

$$

Short term...jrb
long term...jpc
aviation...dwe

National Weather Service Glossary of Abbreviations