Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
600 am CDT Friday may 24 2013
Short term...(today and tonight)
issued at 409 am CDT Friday may 24 2013
Starting out this short term period at 500 mb with a closed low over the
Pacific northwest...sharp trough across eastern noam...with a ridge across the
center of the continent. By Sat morning...the closed low will have
pushed east some...helping nudge the upper ridge east as the eastern
trough moves to the middle-Atlantic coast. As a result...will see
upper ridge axis slide from its current position up across the
central Dakotas over to Minnesota/Iowa by 12z Sat...which will help bring an
abrupt end to the current dry spell...which to this point has lasted
all of one day.
At the surface...cool/dry Canadian high currently centered down across
WI will have moved over to mich by Saturday morning. With Pacific northwest
trough nudging east...will see increasing Lee through throughout the
day today...which will help quickly bring moisture back north across
the plains.
For the short term...big question surrounds precipitation chances starting
this afternoon in the west and spreading east across the rest of the
area through the night. High pressure over WI has provided US with
an incredibly dry antecedent airmass...as evidence by the 00z
sounding from kmpx last night with a precipitable water of 0.14 inches. As we go
through the day...isentropic moistening and lift on the 305/310k
sfcs really picks up on the GFS/NAM this afternoon/evening. This
will result in pretty thick middle-level cloud cover...along with
precipitation generation above 700 mb. The question then becomes can any of the
precipitation make it to the ground...as forecast soundings from the NAM
show an incredibly dry layer of air down around 800 mb that must be
overcome to get anything to the surface. Hi-res cam refl forecasts show
light returns moving across Minnesota during the afternoon...pushing into
WI by 3z...then moving into the far eastern County Warning Area by 6z on this isentropic
lift. Based on the hi-res models...this batch of showers would be
most likely along/north of a Benson to Red Wing line and would be
indicative of a high pop/low quantitative precipitation forecast situation. Thinking probability of precipitation may need
to be increased and moved east faster NE of the Benson to Red Wing
through 06z than the current grids have...while probability of precipitation may be able to
be lowered through 6z south of this line.
After 06z...precipitation potential should shift to south of the Benson to
Red Wing line as strong 925-850 mb moisture transport lays southeast into
Iowa. For this second batch of forcing...the one outlier is the
NAM...which brings the nose of this moisture transport into southern
Minnesota...everything else keeps it down in central Iowa. In addition to
moisture convergence on the nose of this moisture transport...fgen
in the 925-850mb looks to increase as well...which should aid in the
development of thunderstorms around 6z over western Iowa that then track
southeast across Iowa through the rest of the night. Could see a couple of
inches of precipitation overnight in some spots in Iowa...but mainly
light precipitation is expected over Minnesota...with the highest amounts down
along the I-90 corridor. Model blend followed for this second batch
of precipitation was the Gem/ECMWF...which both keep bulk of precipitation on the
nose of the low level jet over Iowa.
For temperatures...did nudge highs down a couple of degrees from
what was inherited...with highs mainly in the 60s today. Will make a
run at 70 across western Minnesota...but feel dense cloud cover by the
afternoon should slow down temperature increases there. For western WI...held
highs there in the low 60s as they will be closer to the cold
trough...plus with lows falling to around 30 this morning...even a
30 degree diurnal swing would hold highs to around 60.
Final thing to watch for the afternoon will be the dewpoints...
especially in western WI. Current dewpoint grids followed a blend of
blended MOS with a mix down to h9 on the NAM...which resulted in
dewps slowly rising into the middle 30s in WI this afternoon.
However...when looking at a NAM forecast sounding for eau this
afternoon...there is that impressive wedge of dry air forecast to
hang out at 800 mb. If the boundary layer can grow deep enough to tap
into that air...then another day with dewps tumbling into the middle
20s can be expected across western WI...which would have significant
implications to minimum humidities this afternoon...possibly
falling back into the teens. With the increasing east-southeast winds...this
will have to watched from the fire weather perspective.
Long term...(saturday through thursday)
issued at 409 am CDT Friday may 24 2013
The Holiday weekend remains filled with cloudiness...precipitation
chances and cool high temperatures. The threat for severe threat
remains very low.
Rain showers and isolated thunderstorms will be ongoing across
eastern Minnesota and west central WI Saturday morning as a short wave
and isentropic lift move through. All guidance is in good
agreement on diminishing the rain during the afternoon with
western areas perhaps being dry. An upper ridge is forecast to
amplify some over US Saturday night and Sunday...shunting short
wave energy coming in from the large western trough to the west
and south of US. This would keep the best precipitation chances
along and south of the Minnesota River Valley... with dry weather over
our far eastern WI counties. The ridge will slip east Sunday night
with several waves of showers and thunderstorms moving through
Memorial Day. Rainfall from Saturday through Monday may amount to
several inches across NE/Iowa where the low level moisture transport
is focused along a strengthening warm front. Rainfall in our area
for the three day period should range from a tenth of two in WI to
an inch in southwest Minnesota. It will be cool through the weekend with
extensive cloudiness and east-southeast low level flow. Highs on Saturday
will struggle to reach the upper 50s to middle 60s. Highs by
Monday should reach the middle 60s to lower 70s.
Things will begin to change on Tuesday as the warm front to our
south begins to lift north...reaching our forecast area for Wednesday and
Thursday. It will turn warmer and more humid with much better
chances for organized strong to severe convection. A check of the
cips analogs valid for next Wednesday indicated widespread severe
reports over our area. Only one of the 15 analogs had no severe
reports in our County Warning Area. The threat for heavy rainfall will be also
be on the increase as well with precipitable water forecast to be at least 2
Standard deviations above normal. The cips analogs also point to
the potential for several inches of rain. By the way... msp has
already had nearly 14 inches of precipitation so far this year...
which is more than 5 inches above normal.
&&
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 600 am CDT Friday may 24 2013
Lots of uncertainty with precipitation potential this taf period...as
strong isentropic lift and warm advection will bring a significant
increase in cloud cover by the afternoon...but precipitation will have to
fight through very dry low levels to reach the surface. Given dry
air...slowed timing a couple of hours compared to what hi-res
reflectivity forecasts show. Whatever is seen...it looks to
sprinkles/light rain...with VFR conditions expected to prevail. What
this rain will do is moisten the low levels...increasing the
likelihood of seeing MVFR or lower ceilings by the end of the night in
Minnesota. May see some thunder with this activity as well...but at this
point it looks isolated and not Worth putting in the tafs. Best
chance for thunder though would be at rwf. For winds...with
increasing gradient...will see southeast winds increase during the day and
do not anticipate much of a drop in speeds overnight.
Kmsp...confidence in forecast high through 00z...but confidence
drops off quickly after that...especially with precipitation chances.
Spcwrf along with the nmm would bring rain in as early as 00z...but
moisture profiles in NAM/GFS soundings would suggest it is closer to
6z before low levels are moist enough to allow precipitation to the ground.
For ceilings...by Saturday morning...confidence is high that sub VFR
ceilings will be in place...just a question of how early. NAM has low
ceilings in around 12z...with the GFS closer to 18z...so split the
difference at this point. For now...kept ceilings above 017...but they
could certainly come in lower than that
/outlook for kmsp/
Sat...chance MVFR or lower ceilings. Chance shra/tsra. Winds southeast 10-15 kts.
Sun...chance MVFR or lower ceilings. Chance shra/tsra. Winds southeast 10-15 kts.
Monday...chance MVFR or lower ceilings. Chance shra/tsra. Winds southeast 10 kts.
&&
Mpx watches/warnings/advisories...
Minnesota...frost advisory until 7 am CDT this morning for mnz045-052-053-
063.
WI...frost advisory until 7 am CDT this morning for wiz014>016-
023>028.
&&
$$
Short term...mpg
long term...rah
aviation...mpg
