Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 152017

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
317 PM CDT Tue Aug 15 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 316 PM CDT Tue Aug 15 2017

An active short term period ahead with the main concern being for
locally heavy rainfall.  The severe weather threat is low this for
this period, but we could see some small hail and a few of the
stronger cells produce some wind gusts.

As of mid afternoon, there was an area of showers and thunderstorms
well to our south in association with the elevated front down across
Nebraska and into Iowa. Upper level flow veers from southerly to
southwesterly, which is trying to push the aforementioned activity
toward our area.  Agree with the HRRR solution in weakening and
potentially phasing this activity out over the next several hours as
it lifts north of the pool of moist and modestly unstable air to the
south.  Reduced pops quite a bit this evening across the forecast
area, as the onset of precipitation has slowed across the general
model consensus which makes sense given current placement and
slower advancement of the unstable airmass into southwestern MN.

Hence, expecting showers and storms to begin developing across
southern MN primarily after 06Z in response to the theta-e
advection, but also the strengthening jet streak to our west and the
shortwave energy advecting into western MN late tonight.
Expecting the highest QPF across western MN in conjunction with
the shortwave energy. In terms of heavy rain and severe potential,
not too concerned for either tonight. Instability and shear will
be limited and forcing is not nearly as strong as it will the
following night.

For tomorrow, the main shortwave will lift out of Colorado tomorrow
morning, with surface cyclogenesis occurring as it lifts
northeast during the day. The precipitation that develops
overnight will still be lifting through early in the morning, but
there looks to be a lull in the activity from mid morning through
mid afternoon as the forcing associate with the main wave arrives
later in the day tomorrow. Dry southeasterly flow will only aid
in the drying things out somewhat by midday. But, as the system
begins taking on a negative tilt and strengthening as it
approaches tomorrow afternoon, showers and thunderstorms are
expected to quickly develop. Deeper instability with this period
justifies a marginal risk as small hail and a some gusty winds
could develop. With PWATs increasing to near 2" and deep skinny
CAPE still shown in the forecast soundings, certainly looks to be
a heavy rain threat more than anything, especially with the warm
front and surface low lifting into the area by late in the day
tomorrow. Expecting this period through tomorrow night will bring
the bulk of forecast rainfall totals.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 316 PM CDT Tue Aug 15 2017

By tomorrow evening, the surface low will have lifted into southern
Minnesota, but there has been some wobbling of this feature among
the guidance, so it certainly bares watching what changes will be
needed in the forecast between now and then.  Generally expect the
low to continue lifting northeast as the upper wave strengthens
overnight, which implies very strong forcing for ascent which should
have no problem producing heavy rainfall across the area. Depending
on the track of the surface low, the warm sector could also see slow
moving segments only raising the risk of locally heavy rain. This
is due to a lack of speed and directional shear late tomorrow

This system will depart to our east Thursday, but still expect some
precip on the backside of the system mainly across eastern MN and
western WI Thursday morning. This system will bring little change
in the actual thermal profile to the region, so as conditions dry
out, we should warm back up to the upper 70s.

Another shortwave looks to move through rather quickly Friday-Friday
night which will likely bring scattered showers and thunderstorms to
the area.  This system will come in from our northwest and be much
weaker and faster moving than the previous system.

Temperatures look to warm into the lower 80s as we get into zonal
flow after the Friday system, with another potential quick moving
and weak shortwave for late Sunday.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1247 PM CDT Tue Aug 15 2017

Fog & low clouds have dissipated across the area with VFR
conditions prevailing for the resat of the day. Widespread
showers & thunderstorms will move northeastwards into the area
late this evening. Ceilings will deteriorate through the
overnight hours with most of the area seeing borderline MVFR/IFR &
the heaviest precipitation beginning around sunrise, & continuing
through the morning

VFR today, with rain moving into the area around midnight.
Expecting precip to begin at the terminal around 09Z, with MVFR
conditions and perhaps a few rumbles of thunder. Conditions
deteriorate to near-IFR mid-morning, with a break in showers &
improving conditions expected during the afternoon.

Wed...MVFR/IFR with -TSRA. Wind S-SE at 5-10 kts.
Thu...VFR. Wind N-NW 5-10 kts.




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