SPC Outlook – 5/14

Thursday SPC Forecast

...Upper MS Valley and western Great Lakes...
   04 UTC surface observations show upper 50s to low 60s dewpoints
   already spreading northwest into the upper MS Valley as a surface
   low begins to organized across the northern Plains. Confidence
   remains high that dewpoints in the mid 60s should overspread much of
   the region by mid-day Thursday and will contribute to regional
   MLCAPE values upwards of 2000-2500 J/kg. Strong forcing for ascent
   along the approaching front and within the left-exit region of a
   50-60 knot mid-level jet will erode capping at the base of a stout
   EML and promote thunderstorm development by early afternoon across
   parts of central to southern MN. Elongated hodographs with
   cross-boundary mean winds will promote organized, initially
   discrete/semi-discrete cells with an attendant severe wind and hail
   (potentially as large as 2 inches) threat. Veering winds through the
   lowest 2 km will also support a tornado threat with early
   convection. 

   With time, persistent strong forcing along the front should promote
   some degree of upscale growth/clustering, though supercells will
   remain likely on the southern fringe of the line where capping will
   remain stronger and favor more isolated convection. The potential
   for somewhat longer-lived supercells may be maximized across
   southeast WI into adjacent portions of IL, northern IN, and
   southwest lower MI. Forecast soundings depict ESRH values on the
   order of 200-300 m2/s2, suggesting a significant tornado threat
   could materialize. Confidence in the coverage of such cells remains
   limited, but trends will be monitored for the need for increased
   tornado risk probabilities.

Weather History

1886: A tornado injured one person and damaged several homes north of Lansing. The injury occurred as a conductor was blown from a train.

2004: A tornado struck the southeast side of Maple Rapids, destroying a silo and damaging two others. A shed was also damaged. A couple of two-by-fours were driven into the side of a pole barn. Shingles were damaged and ripped off two homes.

On May 14, 1987, Oakland County was hit by an F1 tornado at 5:10 PM.

Forecast Discussion

Another day of mostly afternoon showers and thunderstorms today
as the upper low results in some diurnal enhancement of the
showers. Coastal fog is possible near Lake Michigan this afternoon
if winds come onshore.

The main forecast concern remains the severe weather potential
Thursday evening. An upper trough moves east with cyclogenesis
across the Dakotas and the sfc low deepening to near 980 mb as it
moves into Minnesota. An advancing cold/occluded front moves into
the moist and unstable air mass across Lower Michigan traversing
the forecast area roughly between 8 pm and 2 am.

Shear profiles show potential for organized severe with
sufficient mid level lapse rates for deep updrafts and low level
helicity sufficient for rotating updrafts. Main threats are large
hail, damaging downbursts and transient tornadoes. The severe
threat winds down after midnight as the deep moisture exits to the
east.

Friday should be breezy with a good amount of sunshine, though
ensemble members are far from settled whether this will be a day
with gusts around 30 mph or 40-45 mph. A shortwave trough rounding
the base of the low and a proximate cold front may touch off a
quick round of showers late Fri evening, perhaps thunderstorms in
Southern Michigan given the potential for instability rebuilding
to 500-1000 J/kg.

Saturday does have the potential to be another breezy day,
though the magnitude of peak gusts is still questionable among the
ensembles, dependent on the pressure gradient between the low over
the northern Great Lakes and the high over the SE US.

Canadian sfc high builds in early next week with potential for
scattered frost Sunday night and Monday night.