Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1134 am CDT Sat Apr 25 2015
Updated for aviation discussion below
Short term...(today and tonight)
issued at 347 am CDT Sat Apr 25 2015
Both regional radar and water vapor imagery show the main weather
feature in the region rather well this morning and that is the upper
low moving into the Kansas City area. Overnight...we have seen most
of the forcing consolidate down into this upper low...which has
pretty much dried up the precipitation across MN/WI. The one holdout
area for rain is out in wc/SW Minnesota as remnants from convection
that developed Friday afternoon/evening continue to slowly drift
west. At this point...the area of precipitation really is not much more
than a batch of drizzle as stratus has slowly lowered to
under 500 feet.
Main change to the forecast for today was to basically remove any
mentionable probability of precipitation from the entire mpx County Warning Area from the morning onwards.
We will likely see some -dz continue through about 15z out
west...but other than that...advection of very dry air from high
pressure north of Lake Superior looks to win out. With all 4 members
of the hopwrf showing precipitation the rest of today getting about as far
north as I-80...feel confident in dry weather persisting to trend
the forecast dry the rest of today.
Though the dry air will be keep the deep moisture and precipitation at
Bay...it will be a while before we lose the clouds. The back edge of
the clouds was still working across the western u.P. Of mich at 3 am and
will take a while to get out here. H850-700 mb relative humidity from the rap/NAM was
used as a proxy for moving this clearing from east to west across
the area. This clearing does not look to get into the east end of
the mpx County Warning Area until about 21z but will propagate west across the area
fairly quickly this evening as it will be aided be the loss of
heating with the setting of the sun. Expect skies to be pretty much
clear area wide shortly after midnight.
For temperatures...the cloud cover today will keep temperatures
fairly uniform in the middle 50s...though if the clearing moves
faster than currently expected...we will likely see highs in western WI
jump into the 60s. For tonight...clear skies along with the very dry
airmass moving in will lead to excellent radiational cooling
conditions...with lows back into the upper 20s expected to the north
Long term...(sunday through friday)
issued at 347 am CDT Sat Apr 25 2015
Continuity between the European model (ecmwf)...GFS and Canadian has begun to wane
some for next week over what we saw the past two nights. The
European model (ecmwf) is more progressive with a northern stream short wave moving
through Monday night and Tuesday along with another northern
stream feature for late in the work week. The culprit is a large
upper low over New England that will move only a little eastward
over the next 5 days. This is the same low that was northeast of
Minnesota/WI back at the beginning of the week along with all the wind.
The Canadian and GFS are trending more slowly with the short wave
Monday night and Tuesday along with a more southerly trajectory
across the eastern Dakotas and western Minnesota versus the European model (ecmwf) which
pushes the feature southeast across the upper Mississippi Valley.
There is not a very impressive return of low level moisture ahead
of this feature as well due to a strong southern stream low moving
across the Southern Plains and Gulf Coast states. Therefore...kept
probability of precipitation to slight chance for Monday night and Tuesday. In the wake of
this feature....large scale ridging will be building over the
central and northern plains along with an infusion of warm air in
the low levels. By Friday afternoon 850 mb temperatures range from 10 to 15
degree c across the County Warning Area. However...The Fly in the ointment is a
northern stream wave on the European model (ecmwf) moving across the northern
plains and upper Mississippi Valley region helping to initiate
some showers and thunderstorms. This is not seen in the GFS or
Canadian with ridging prevailing along with high temperatures
reaching the 75 to 85 degree mark. For now...used a 50/50 blend of
GFS mix-down from 850mb and crextend for the highs on Thursday
with a 25/75 blend for Friday. The lower blend on Friday accounts
for the possibility that the European model (ecmwf) is catching on to something
here. Otherwise...the beginning of the long term...Sunday and
Monday...will offer some fine Spring weather with high pressure
ridging across the area. The origin of the high is from Ontario so
a dry (low humidity) northeast flow will cover the region. Highs
will actually be a little above normal...after a week of below
normal readings. Highs Sunday will be from around 60 to the middle
60s with highs on Monday in the middle to upper 60s.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1134 am CDT Sat Apr 25 2015
High confidence of VFR conditions through the period. Dry east
flow eroding clouds this morning. Should see most of the lower
4-5k feet ceilings gone from the far east by 18z...with some low end middle
level clouds from 7-9k feet moving in from the north. These may
brush wc wis area into the afternoon. VFR ceilings to the west should
erode through 02z sun...with area becoming sky clear by 05z. East wind
10g19kts over the area into the afternoon with mix...then becoming
light east/NE into Sunday.
Kmsp...VFR through period. East winds a bit gusty with full mix
through the afternoon. Then becoming more NE into Sunday.
/Outlook for kmsp/
sun...VFR. Winds NE 5-10 knots.
Monday...VFR. Winds north-northeast 5-10 knots.
Tuesday...VFR. Winds northwest 5-10 knots.