Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 160028

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
728 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 bears 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 night.

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 00Z TAFS through 00Z Wednesday evening)
Issued at 728 PM CDT Tue Aug 15 2017

Concerns are a couple rounds of precip with IFR/MVFR for much of
the period. Forecast this evening starts out with just incoming
mid and high clouds, then scattered showers begin moving in and
developing late tonight, especially over the MN sites into far
western WI. There is some thunder risk late tonight, but the main
story will be the development of IFR ceilings over much of the
area. Confidence is rather high in this regard. Ceilings should
only rise to MVFR or maybe low VFR by mid afternoon tomorrow,
since a southeast wind with continuous moisture flow along with
inversion will serve to keep MVFR ceilings tomorrow afternoon.
Then storms develop later afternoon, but placement is tough at
this point.

Short term models have continued the trend of delaying onset of
precip a bit, but most recent indications are that some showers
should move in around 09Z or so, with a drop to IFR a few hours
thereafter. Confidence is above average that we`ll get some
showers and IFR, but not at all sur ethat we`ll have thunder. And
confidence in the timing is only average. After showers depart
and ceilings improve to MVFR, the MVFR may well persist all
afternoon due to southeast flow and inversion.

Thu...VFR. Wind N-NW 5-10 kts.
Fri...VFR. Chance afternoon/evening TSRA. Wind SW 5 kts.
Sat...VFR. Wind W 5 kts.




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