Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1135 am CST Wednesday Dec 17 2014
Short term...(today and tonight)
issued at 348 am CST Wednesday Dec 17 2014
Quiet weather this period with main focus being the extent of the
low cloud cover /and subsequent snow showers/ and the temperatures.
Early this morning...water vapor imagery easily depicted an upper
low advancing southeastward through northern Wisconsin. Significant
middle level drying was observed directly through The Heart of the
forecast area...but the real concern is whether or not the low levels are
saturated. The low clouds overnight have been a little less
widespread than expected...with a solid clearing line from
Alexandria through St cloud...to near New Richmond Wisconsin...with
clear skies southwest of this line at the current time.
Temperatures have hence dropped into the single digits across most
of western Minnesota while temperatures remain in the teens further
northeast thanks to the cloud cover there. The upper low will
continue rotating southeastward toward Lake Michigan /north of
Chicago/ today and the stratus in northeastern Minnesota will advance
southward due to the counterclockwise circulation of the upper low.
Its not as simple as just timing in the clouds to southern Minnesota
however...because continued subsidence has proven it can scour out
some of the cloud cover. While this is occurring...a surface high
will be pushing in from the west. Model soundings indicate a fine
line between overcast and clear skies...with a dew point depression
of 3 degrees being all that may be required to see the clouds
scatter out. So...as the clouds continue south...scattering should
occur. Southwestern Minnesota should remain mostly clear to partly
cloudy...while Glenwood through the Twin Cities may be in that
transition zone this morning...and it looks like a good bet that
clouds will linger most of the day to north and east of this line.
The areas that do see cloud cover can also expect to see flurries or
a few snow showers similar to yesterday afternoon. With the high
pushing in from the west...we're already seeing wind speeds drop due
to the weakening pressure gradient out west. This trend will only
continue through tonight as winds become calm. With the calm
winds...sky cover tonight will be important in predicting the
overnight lows. Went with a conservative forecast in eastern Minnesota and
western WI in regards to temperatures because clouds may still linger. Not
confident in that though due to the shallow layer of saturation
expected. In addition...the NAM shows a strong signal of saturation
under 3k feet...but the GFS and ec are more optimistic in regards to
cloud cover. If skies do clear...low temperatures may be a few degrees
cooler than currently advertised.
Long term...(thursday through tuesday)
issued at 348 am CST Wednesday Dec 17 2014
The long term is chock full of clouds and temperatures near or
above average. Benign weather will continue until next week when
the pattern becomes more active...including the likelihood for a
clipper early in the week...possibly morphing a large and strong
system somewhere nearby for Christmas evening and day.
Surface ridging will slide a bit to the east Thursday and Friday
with return flow gradually strengthening. Considerable low clouds
are currently in place to the west of this ridge over much of the
western half of North America. As low level moisture increases
late week...think this stratus will become entrenched across the
area again with Little Hope to see much sun through the rest of
the period. Although usually too moist...the NAM forecast
soundings indicate a little freezing drizzle potential Friday. Quantitative precipitation forecast
is very light...but notable on all global models into Saturday.
Think potential is too low to mention at this point but it will be
Energy will shift from the southern stream to the northern
stream early next week as a strong 590+ dam middle level ridge builds
off the coast of California. One such piece of energy will arrive
Monday as a trough digs into the plains downstream of the building
ridge. A pretty chaotic and discombobulated system will probably
not bring much meaningful precipitation to the upper Midwest...but
cyclonic flow and low level moisture will produce occasional light
snow/flurries...or drizzle/freezing drizzle depending on the
The evolution of this system and trough gets more interesting at
the end of the period Tuesday as it intersects a stalled frontal
boundary over the Gulf. The result is a consolidation of energy
and a lot of moisture being ingested into the system midweek -
fueling a rapidly deepening system somewhere over the Great Lakes
or East Coast near Christmas evening or day. This would be a high-
impact system and certainly one to watch.
Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1132 am CST Wednesday Dec 17 2014
Scattered flurries remain in place over eastern Minnesota and western
WI...associated with an upper low over the western Great Lakes.
This feature is drifting eastward...but enough low level moisture
and forcing remain for scattered MVFR ceilings and flurries through
the rest of the afternoon. Low clouds should mainly dissipate by
tonight...with VFR conditions and light winds expected at most
terminals through the end of the period. However...scattered low
clouds can not be ruled out tonight through Thursday morning since
decent low level moisture remains in place.
Kmsp...periods of MVFR ceilings are possible the rest of the afternoon
as scattered/broken low clouds move through with a few flurries.
Most low clouds should dissipate by the early evening...with VFR
conditions expected to prevail from the evening through the
remainder of the period.
/Outlook for kmsp/
Thursday...VFR. Winds variable less than 5 kts.
Friday...VFR. Winds south-southeast 5 kts.
Sat...VFR. Winds south-southeast 5 kts.