Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1028 PM CDT Tuesday may 5 2015
Update...for 06z aviation discussion below
Short term...(this evening through wednesday)
issued at 303 PM CDT Tuesday may 5 2015
Surface analysis shows a weaving warm front extending eastward from the Kansas/Colorado
area across the middle-miss River Valley region into the Northern Ohio
valley region. Aloft...a northwest-southeast oriented ridge axis extends southeastward
from Manitoba province through the Dakotas and into the southeastern Continental U.S..
along and just ahead of this front is low clouds with isolated rain
showers. Going into the evening and overnight hours...both surface and aloft
features will steadily shift northward...bringing together a deeper southerly
flow in the wake of each feature. However...the lack of
convergence at the surface and baggy flow aloft will spell only isolated-
scattered -shra coverage for southern and western Minnesota overnight through tmrw morning.
In addition...there still is plenty of blyr drier air that will
need to be overcome to have measurable precipitation so quantitative precipitation forecast amounts will
be rather unimpressive...which has been the trend in short-term
model runs through the day. Once these features lift into central and
northern Minnesota by tmrw morning...a more poignant upper level trough axis from the
northern plains through the middle-miss River Valley will lift northward...and this
positive vorticity advection will aid in creating not only increased coverage of showers but
also better potential for a few thunderstorms from time to time during the
day. Thus...have shown probability of precipitation in the 30-50 percent range for tonight
followed by probability of precipitation in the 50-70 percent range for tmrw. Severe weather
potential also looks rather limited due to the lack of
insolation/instability and meager lapse rates. Speaking of the
lack of insolation...the northward moving clouds this afternoon have toyed
with maximum temperatures in far southern Minnesota. Temperatures held in the middle-upper 50s around
the I-90 corridor while western and central Minnesota had temperatures hitting the
low-middle 70s. The blanket of clouds will keep temperatures in the 50-60
degree range followed by highs in the low-middle 70s area-wide tmrw.
Long term...(wednesday night through tuesday)
issued at 303 PM CDT Tuesday may 5 2015
Wednesday night through Friday morning: a large upper level ridge
will persist across central US into southern Canada from Wednesday
night into Thursday. However...a potent middle-level shortwave will
move from the northern plains states and into Minnesota during this
period. Ahead of this feature...deep southerly flow will bring warm
and moist air into our area from Wednesday night into Thursday.
Combined with an elongated surface low moving through the eastern
Dakotas and northern Minnesota...this setup will lead to multiple chances
of precipitation during this period.
From Wednesday evening through Thursday morning...a warm front will
move northward through our County Warning Area...providing enhanced forcing along
with positive vorticity advection ahead of the approaching middle-level
shortwave. The best precipitation chances will be in western
Minnesota...where the strongest forcing will reside. Went with likely probability of precipitation
in this region...and chance probability of precipitation elsewhere.
By Thursday afternoon...the shortwave and surface low will move into
western Minnesota...with southern Minnesota and western WI in the warm sector.
Not sure how unstable the atmosphere will get with widespread cloud
cover and ongoing showers and storms possible in the morning and
early afternoon...but there is the potential for strong storms
during the late afternoon and evening ahead of the cold front moving
into Minnesota. The 12z NAM brings cape up to 2500 j/kg into the Twin
Cities area by 00z on Thursday evening...while the GFS limits cape
values to 1000-1500 j/kg at this time. Combined with 0-6 km bulk
shear values from 20-30 kts...forcing from the shortwave and cold
front...and surface dewpoints in the lower 60s....thunderstorms will
be likely from Thursday afternoon through the evening hours...and
severe weather cannot be ruled out in eastern Minnesota and western
WI...especially if the cape values shown in the NAM occur over these
Chances for strong to severe storms will diminish Thursday night as
instability decreases. However...the cold front will be slow to move
through our area...and enough moisture and forcing will persist to
maintain chances of showers and thunderstorms across the region
through early Friday morning.
Friday through Monday...aforementioned wave will be departing to
the northeast...while an upper low strengthens over The Four
Corners region through Saturday. High pressure to our north will
keep US dry for Saturday...with mainly northeast winds of around
10 miles per hour by the afternoon. This next low will lift northeast through
Monday...and looks to spread widespread rain into southern Minnesota by
Sunday evening in a warm air advection regime. With the surface low expected to
track to our south...we'll generally be on the cool/deformation
side of this system. So...temperatures look to fall below normal
Sunday and continue through at least Monday.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1030 PM CDT Tuesday may 5 2015
Ceilings and visibilities will continue to degrade tonight as a warm front
lifts north across the area. Scattered rain showers across
central Minnesota/WI will be possible into the early morning hours...but
more widespread showers and thunderstorms are expected to spread
into the western area on Wednesday afternoon...and expand east-
northeast across the area during the evening. IFR ceilings will become
prevalent between 06z and 09z. Visibilities should be primarily 3sm or
greater...with patchy fog possible overnight. Should see
temporary improvement on Wednesday morning as the first wave of
showers lifts north of the area...and then the next batch moves in
from the west during the afternoon...this time with thunder
possible. Southeast winds through the period...with gusts
diminishing tonight then redeveloping on Wednesday morning.
expect IFR ceilings /below 1000 feet/ through the overnight...with
patchy br and showers remaining isolated at best. More widespread
activity expected by 00z Thursday...with isolated thunder possible.
/Outlook for kmsp/
Thursday...MVFR conds likely with IFR conds possible in shra/tsra.
Winds S 17-25 kts.
Friday...VFR. Winds northwest 7-10 kts.
Sat...VFR. Winds NE 10-15 kts.