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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
733 am CDT sun may 24 2015

Update...
issued at 700 am CDT sun may 24 2015

Updated to include 12z aviation discussion below.

&&

Short term...(today and tonight)
issued at 345 am CDT sun may 24 2015

Today and tonight...surface analysis depicts several weak low pressure
centers over the Southern Plains while a deep upper level low rotates over
the 4-corners region /showing up nicely on WV imagery this mrng/.
In addition...a large subtropical ridge over the southeastern Continental U.S. Will
be a major player in forcing copious amounts of western Gomex moisture
into the upper miss River Valley over the weekend. This deep upper level
low will also help the conglomeration of weak surface lows into an
organized surface low pressure system later today...and help slowly nudge it
slowly north-northeast through tmrw. Isentropic lift heightens greatly this morning
in advance of this system...helping spread precipitation currently
moving into far southwestern Minnesota northward and encompassing much of the weather forecast office mpx
County warning forecast area by midday today. Have gone along with the consensus of the
hopwrf/hrrr/sref/arw-E in slowing down the initiation of the
precipitation into the County warning forecast area...more towards the 12z hour for southern portions
and to 17z for the entire County warning forecast area to be in the likely pop range. Much
of this initial precipitation surge looks to be with a pre-warm-frontal
trough which shows up in the 700 mb through 500 mb pressure levels...indicating a
midlvl disturbance riding around the eastern periphery of the deep
low. Once this disturbance moves through...the models do show a
diminishing of the precipitation coverage if not a temporary break in the
action. Have depicted this by a lowering of probability of precipitation back to the chance
category approaching 00z this evening. More widespread precipitation
commences by late this evening into the overnight hours as the warm front
itself pushes into Iowa tonight and nudges up against the Iowa/Minnesota
border by daybreak Monday morning. There is the potential for pockets of
heavy rain as precipitable waters increase to around 1.50 inches later today through tonight
and the overall progress of this system continues to be on a
slowing trend. As for convective potential... the continued lack
of instability as evidenced by no Bona fide periods of sunshine
and meager prognosticated convective parameters will mitigate good
thunderstorm potential. Cannot rule it out completely so have
maintained slight chance thunder for this afternoon through tonight but any thunderstorm
development is not expected to be strong/severe. As for
temperatures...have nudged temperatures down slightly from inherited
values...especially over southern Minnesota...due to extensive cloud cover and the
lack of insolation. In some cases...maximum temperatures may well have
already been reached and temperatures may remain steady or even fall a
degree or two throughout the day today. That said...min temperatures early
Monday morning are expected to be warmer than current values this morning
due to slight warm air advection with the approach of the warm front late tonight and
deep southwesterly flow from the upper level pattern. Will look for lows in the
55-60 degree range Monday morning.

Long term...(monday through saturday)
issued at 345 am CDT sun may 24 2015

Monday will start off wet with the surface low lifting north
through north central Iowa and deformation zone precipitation
expanding across much of the forecast area. At this time...expecting a band
of heaviest precipitation just on the back side of the surface low
through Monday afternoon...but there remains variability in the
guidance on where exactly this band sets up. The 00z runs of the
GFS and NAM came in with good agreement that the area just west of
the Twin Cities through the Mille Lacs Lake area could see the
heaviest amounts on Monday /upwards of an inch/.
However...confidence must be somewhat limited because the
ec...which has been one of the leading models in the handling of
this system as it has come closer in time...is indicating a
surface low track farther east through western Wisconsin and wants
to place that heaviest band of precipitation from Red Wing through New
Richmond. In our forecast area...the most uncertain area for precipitation is
western WI at this time...because if the GFS/NAM solutions end up
being more accurate...that could introduce a dry slot over Eau
Claire and really limit precipitation there. So...much of how Memorial
Day turns out will depend on the track of the surface low. As is
usual with surface lows...areas east of the track will be mainly
limited to the warm air advection precipitation band which typically move
through quickly...then battle the dry slot. Meanwhile...areas
immediately west of the surface low could see several hours of
decent rainfall rates and over an inch is certainly possible.

Strong ascent through Monday morning on the back side of the surface
low is supported by the very strong differential divergence
indicated by the deterministic guidance as we are in a prime region
of right entrance region induced lift of the upper jet. Some
instability and precipitable water values exceeding 1.5" support the idea of
seeing some decent rainfall totals in that deformation band that
will pivot in the vicinity of eastern Minnesota and western WI. Much of the
area will likely dry out Monday afternoon...especially South. West
central Minnesota however may still be plagued by light precipitation right
through the afternoon. By Monday night...the low will have moved off
into the northern Great Lakes and the forecast area may have a few scattered
showers. It looks like a second trough and associated surface low
will then push into the Midwest but most of the precipitation associated
with that feature should remain south of the forecast area. So...cut probability of precipitation back
Monday night.

The track of the second low should remain southeast of our area
through Wednesday...but it may spread light precipitation as far west as
eastern Minnesota and western WI...so have included some low probability of precipitation Tuesday
during that time. Expecting a dry day Wednesday before more probability of precipitation
come back Wednesday night through Friday. A messy pattern indeed
for the latter half of the week as a modest feed of moisture and
instability looks to give broad chances for thunderstorm
development during that period.

Little change for the end of the week and next weekend. The ec/GFS
still agree that a Continental polar air mass from the north will
push into the upper Midwest with cooler and drier conditions
expected. However...nailing down the exact timing of the frontal passage
is tough this far out.

&&

Aviation...(for the 12z tafs through 12z Monday morning)
issued at 700 am CDT sun may 24 2015

VFR decks across all sites at initialization. -Ra starting to
overspread S and SW Minnesota and will continue to move north through the
morning hours. Much of this activity looks to remain as VFR but late
morning into early afternoon there is a period where heavier rain with
MVFR ceilings may impact the terminals so have included a tempo group
to advertise this. Then...following a short-term model
consensus...there looks to be a late afternoon-early evening diminishing if
not break from the precipitation before more impressive warm air
advection rainfall comes late this evening into tonight. More
widespread MVFR ceilings and visibility are possible then. Lamp guidance
does advertise IFR ceilings this evening through tonight...but am
hesitant at this time to go that far so have bottomed out conds at MVFR.

Kmsp...conds to deteriorate as the day progresses but still mostly
within VFR range. The 4-hour tempo window for MVFR conds could be
adjusted an hour either way depending on how the rain
evolves...but am not expecting conds to go into IFR range as much
of the rainfall will be light. Worse conds are expected tonight...and
ceilings may drop into IFR range for a good portion of the
overnight period. Winds to remain from the southeast throughout the
period with speeds around 10 knots.

/Outlook for kmsp/
Tuesday...VFR. Chance shra/-tsra. Winds SW 10 kts.
Wednesday...VFR. Winds SW 5-10 kts.
Thursday...VFR. Chance shra/-tsra. Winds southeast 5-10 kts.

&&

Mpx watches/warnings/advisories...
Minnesota...none.
WI...none.
&&

$$

Update...jpc
short term...jpc
long term...speed
aviation...jpc

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