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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
603 am CDT Wednesday Aug 27 2014

Updated for 12z aviation discussion...
issued at 557 am CDT Wednesday Aug 27 2014

&&

Short term...(today and tonight)
issued at 338 am CDT Wednesday Aug 27 2014

Early morning water vapor imagery together with GFS 500mb winds
revealed a fascinating example of large scale laminar flow breaking
down into turbulent upper level waves. Although this has no bearing
on the weather across the upper Midwest it is at least Worth
mentioning. The feature of interest is over the Gulf Stream...where
just 36 hours ago an elongated strip of positive vorticity prevailed
between the poleward transport of moist air tied to the outflow of
Cristobal...and the equatorward transport of dry air just off the
eastern Seaboard. By this morning this strip of positive
vorticity had congealed into three distinct areas of cyclonic
circulation that will March off harmlessly into the North Atlantic.

Meanwhile closer to home a low amplitude shortwave was strong enough
to produce light rain mainly north of I-94 overnight. This rain will
continue east this morning. A more vigorous pv anomaly currently
located over eastern Utah will head towards the Midwest and this
will be the focus for shower and thunderstorms late Wednesday night
into Thursday morning. Slowed down the timing of the probability of precipitation close to
the trend depicted in the 27.00 nsslwrf. Although the timing of the
preicip is slightly delayed...the increasing environmental moisture
remains on track. Precipitable water will increase to over 2in across Iowa this
evening and overnight which is a result of low level moisture from
the south combining with moist middle/upper level air tied to the
aforementioned pv anomaly. This will set the stage for heavy rain
once the precipitation does begin early Thursday morning.

Long term...(thursday through tuesday)
issued at 338 am CDT Wednesday Aug 27 2014

Precipitation should be ongoing Thursday morning across at least
the southern half of the forecast area in advance of the
aforementioned pv anomaly /and surface low and warm front/. The
forcing at middle-levels with this system is impressive. Strong
925mb-700mb fgen lifting north across Iowa with strong low level
moisture advection and an advancing upper trough should set the
stage for widespread moderate to heavy rain across Iowa and southern
Minnesota. Precipitable waters of over 2" are forecast across portions off south
central Minnesota by 18z Thursday. Forecast soundings show saturation /or
near saturation/ from the surface to over 500mb with freezing
levels between and 14-15kft and tall skinny cape /at least off the
NAM/. There is no doubt there will be high rain rates and
widespread 1-3" across portions of northern Iowa with this setup
/locally more than 3"/...and this will likely spill over into
southern Minnesota. We are still somewhat uncertain on how far north to
take the heavy rain into MN/WI...because the deep moisture and
surface features remain a bit farther south off the 27.00z GFS
/ECMWF to a lesser degree/ as compared to the NAM/Gem/sref
solutions...which develop a surface low farther north. We will
maintain the 70-90% chance of rain with a heavy rain mention on
Thursday. We might even go back and raise the quantitative precipitation forecast yet this
morning...based on some of the 00-06z guidance.

Beyond the heavy rain potential on Thursday...most of the forecast
area looks like it will remain dry until another front comes
through Sunday evening into Monday morning. The exception will be
areas in southeast Minnesota and western WI...which remain close to the
boundary that passes by on Thursday. There should be 1-2 more
waves tracking along that front from Thursday night into Saturday
morning...however...we think most of the heavy precipitation will remain
just east of the forecast area during this time. That being
said...the evolution of the pv feature will be dependent on how
the deep convection evolves. Widespread storms a bit farther north
on Thursday could keep the boundary closer to the forecast area
and thus increase our chances for rain on Friday and early
Saturday. No Big Warm-ups expected through next week...near to
slightly below normal temperatures are expected due to the influence of
longwave cyclonic curvature across the north central Continental U.S. Into
next week.

&&

Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 557 am CDT Wednesday Aug 27 2014

VFR conditions with light winds. A few middle/high clouds today will
give way to overcast skies late tonight as thunderstorms develop
across Iowa and the cirrus shield spreads across the region.
Storms could impact krwf before 12z Thursday...and the remaining
sites thereafter. Heavy rain and lightning are the primary
threats.

Kmsp...
no significant changes from the general discussion. Could see IFR
conditions with the thunderstorms Thursday morning...but
confidence is too low in the timing to include it in the tafs.

/Outlook for kmsp/
Friday...VFR. Slight chance of MVFR/tsra. Wind east at 5-10 kts
Sat...VFR. Wind north-northwest at 5-10 kts.
Sun...VFR with MVFR/thunderstorms and rain late. Winds south-southeast at 10kts.

&&

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

$$

Short term...jrb
long term...clf
aviation...jrb

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