Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
104 PM CDT Friday Oct 24 2014
Short term...(today and tonight)
issued at 314 am CDT Friday Oct 24 2014
Main concern today is dense fog and the possibility of holding
down temperatures in the southeast County Warning Area due to another shortwave and associated
moisture moving across the area.
First...S/SW boundary layer winds began to increase shortly before
midnight which led to most of the moisture/fog formation in ec/southeast
Minnesota to halt. These winds will also keep the bulk of the lower
clouds/fog across wc WI this morning and therefore...the dense fog
advection has been dropped in Minnesota /with the exception of Goodhue cty/.
Even in wc WI...the dense fog has been limited to the higher
terrain or along the river valleys. Most of the observation were noting
low ceilings...but visibilities were at or above 1sm. The SW/S winds in the boundary
layer will also lead to more moisture surging north/NE across SC/se/ec
Minnesota by middle/late morning...and across most of wc WI during the afternoon.
This will keep temperatures from rising too much...with 60s a better
scenario. Elsewhere...upper 60s/lower 70s remain reasonable with
near 80 in the typical areas of wc Minnesota with a slightly stronger
downslope wind component along the Buffalo Ridge.
There remains some uncertainty with -shra/sprinkles in southeast/SC Minnesota
during the middle/late afternoon hours. Although moisture will be limited to
the first 10k...warm air advection in this layer may allow for some isolated rain showers to
develop. Will keep conds dry at this time...but something to look
at for later this morning.
No changes overnight with skies rapidly clearing from the west as a
potent storm system sweeps across south central Canada.
Long term...(saturday through thursday)
issued at 314 am CDT Friday Oct 24 2014
The long term period starts and ends calmly. It/S the middle
portion that is unsettled due to a potent incoming longwave trough
from the West Coast that will shift into the central
Continental U.S....bringing a storm system with it.
Saturday will start with a large ridge that will encompass much of
the lower 48 states...originating from the Southern Plains. This
ridge will hold its own...shifting slowly eastward through Sunday and
allowing its axis to move across the Mississippi River during the day
on Sunday. It will break down somewhat...but not to the degree
previously depicted on models so the cooling associated with
today/S cold front will not be as strong. There will be very little cloud
cover on Saturday followed by mainly high clouds on Sunday with
highs both days in the low-middle 60s. The ridge will also help slow
the progression of the aforementioned storm system expected to
move across the region early next week.
Getting into the details...a longwave trough will move onshore the
Pacific northwest during the day on sun while a weak area of low pressure develops
in the Lee of The Rockies. A stronger low pressure center will move
eastward in advance of the trough just north of the Canadian border. The
trough will shove the Canadian /nrn stream/ low eastward while picking
up The Rockies /srn stream/ low. The Rockies low again looks
weaker in consecutive model runs...making the Canadian low the
main feature but still with surface fronts dropping into the central
Continental U.S. From it. The conglomeration of these low pressure centers will
shift across Minnesota/WI Monday through Tuesday morning. In addition...the upper level
trough becomes more compact and slightly negatively tilted by
daybreak Tuesday...allowing for slightly enhanced lift for better
precipitation generation. Problem is...there are still model-to-model
disagreements in the intensity of this system due to the
difficulty of handling the phasing...or rather non-phasing... of
the northern and southern streams upstream from the mpx County warning forecast area. This makes
timing and coverage of precipitation a bit problematic. Confidence is
higher that the precipitation will be delayed so have removed probability of precipitation from
Sunday but maintained probability of precipitation no higher than chance range Sun night through
Tuesday. Within that range...best timing for precipitation looks to be Monday
night...especially over eastern Minnesota into western WI...as the upper trough deepens and
shifts the southern stream low through the area.
The upper trough and its associated low pressure centers then shift through
the region by Tuesday evening...followed by prevailing zonal /slightly
WNW-ESE/ flow aloft and a non-descript surface pressure pattern. This will
result in generally precipitation-free weather conds with partly cloudy skies
for the middle of next week. It will also result in noticeably
cooler conds for the last week of October. Highs will drop to the
upper 40s to lower 50s for the middle of next week while lows drop to
30s for the middle of next week...and possibly the 20s by the end
of next week.
Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1245 PM CDT Friday Oct 24 2014
Surface trough at 18z was near a mkt/msp/rzn line with a tongue of
MVFR ceilings racing north to the east of it. Rap/hrrr have had a good
handle on these ceilings this morning...so this forecast favors trends
from those models closely. In the wake of the surface trough...drier
air is moving in...so that will keep any stratus/fog out of the
area in its wake the rest of this taf period. Winds will be
strongest along/north of I-94 Saturday as a surface high GOES from SW
Minnesota into Iowa.
Kmsp...MVFR clouds showed up at the same time as the surface
trough...so confidence is high that msp will be back to VFR and
VFR for good by 20z.
/Outlook for kmsp/
sun...VFR early. Chance MVFR/rain showers in evening. Winds southeast 10 kts.
Monday...MVFR ceilings likely. Chance rain showers. Winds becoming west-northwest 5-10 kts.
Tuesday...chance MVFR/shra. Winds west 15g25 kts.