Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
608 PM CST Thursday Nov 20 2014
Short term...(this evening through friday)
issued at 320 PM CST Thursday Nov 20 2014
Early afternoon water vapor imagery together with 500mb heights and
winds identified the 3 main players that will impact the upper
Midwest this weekend and early next week. The first was a shortwave
off the California coast...which is forecast to translate
southeastward over the next 36 hours. The next is a powerful jet
off the northwest coast...which will begin to come on shore by 00z
Saturday. The third is the shortwave in the left exit region of
this jet which again will be just off the British Columbia coast
by 00z Saturday.
In the mean time...high pressure currently in place across the upper
Midwest will gradually slide southeast. This will set the stage
for ample radiational cooling and another cold night across the
forecast area with most locations dropping below zero...especially
in western WI. By Friday morning southerly flow on the backside of
this departing high pressure will advect moisture across the
region. Forecast soundings show stratus developing from southwest
to northeast. High temperatures will generally be in the upper
20s...which should bring the consecutive days of sub freezing
November highs at msp to 12...making it second only to 15 back in
Long term...(friday night through thursday)
issued at 320 PM CST Thursday Nov 20 2014
Long term (fri night-thu)...not much change in forecast reasoning today
as model solutions are becoming agreeable...with a slight eastward
shift on sun/Mon. With that said...there are still multiple
challenging periods as a large storm system takes shape over the
High pressure will be well to our east by Friday night with return flow
already well established. There is a weak shortwave in the northern
stream that may produce a little light quantitative precipitation forecast overnight Friday...and with
temperatures slowly warming through the 20s...the favored p-type would
likely be -fzdz. This threat should be confined to locations along
and east of Interstate 35. With the warm air advecting over the
snowpack...fog is also a distinct possibility.
There looks to be a break in precipitation for much of the day Sat and the
first half of Sat night as low pressure develops in Texas in advance of
a digging trough. Meanwhile...a second (and weaker) low will be
moving across the northern rockies at the same time. Temperatures will warm
considerably Sat/Sat night as warm air and moisture gets drawn north.
Model guidance is showing temperatures into the 40s on Sat and remaining
above freezing Sat night. Sat night (at least the second half) will
once again feature -ra/dz and patchy fog most prevalent in east MN/WI.
We will continue to see -shra at least in east Minnesota & WI for the first
half of Sunday as a consolidation of low pressure centers begins to
occur over the southern Great Lakes.
The merging of these two lows will cause a rapid intensification of
a singular low somewhere in the Great Lakes early Monday. If nothing
else...this rapid deepening of the low will produce strong winds for
our area. Cold air will quickly wrap around the system by Monday with
any precipitation changing to snow. In terms of precipitation...there will be a
deformation area of precipitation stretching behind the low enhanced by the
upper level low moving directly over the State of Minnesota on Monday. At this
time it looks like the majority of (significant) snowfall will be
confined to the northern half of Minnesota...but there remains a decent
chance for snow over the entire region. Probability of precipitation at this juncture will
remain in the chance category with snowfall amounts just too
difficult to nail down. As mentioned earlier...wind will be the main
impact from this system with sustained winds greater than 20kt with
gusts potentially exceeding 35kt. Depending on the composition and
amount of snow...blowing and drifting snow may become a significant
factor. Please stay tuned to future forecasts as we refine snow
This system winds down for US by Tuesday afternoon with more below average
temperatures and minimal precipitation chances through Wednesday. There is the threat for
more -sn on Thanksgiving...but at this point model spread is pretty
significant so only small chances are warranted.
Aviation...(for the 00z tafs through 00z Friday evening)
issued at 542 PM CST Thursday Nov 20 2014
VFR conditions will prevail at all terminals tonight...with MVFR
ceilings developing Friday morning in our southwestern regions and
spreading northward during the afternoon. Westerly winds have
diminished to less than 10 knots tonight...and will become southerly
by Friday morning. Southerly winds will increase to 10-15 knots by
Friday afternoon...with highest speeds and gusts above 20 knots
possible at axn and rwf.
Kmsp...VFR conditions will likely continue through tonight and
Friday morning...with ceilings gradually lowering to the MVFR range by
Friday early evening. Winds similar to general discussion...with
southerly winds up to 15 knots possible Friday afternoon. Some patchy
freezing drizzle is possible Friday night, but confidence is not
currently high enough to include in the taf.
/Outlook for kmsp/
Friday night...MVFR/IFR and fzdz possible. Winds S at 10g15 knots.
Sat...MVFR possible. Winds south-southwest 10g20 knots.
Sun...MVFR/IFR with fzdz/rain possible. Winds southeast 10 knots.
Monday...MVFR/IFR with -sn/blsn possible. Winds northwest 25g30 knots.