National Weather Service United States Department of Commerce

Forecast commentary -

Near term / through 6 am Saturday morning. Aloft, upper-level ridging is in place. Generally light
south to southwest flow continues to prevail with the return flow around the high. This return flow
has allowed for a surge in midlevel dry air across the region, which has resulted in dry weather and
sunny skies. Temperatures have warmed well above normal for this time of year, with temperatures
warming well into the 70s for most areas, and even upper 70s near us 23.

Upstream, an increase in high cloud cover was noted across the Mississippi River Valley in
association with a deepening longwave trough which will impact the region during the short term
period ( more in the short term section of the discussion ). This cloud cover will begin streaming
into the area tonight, and combined with a lingering southerly wind due to a tightening pressure
gradient, will help keep low temperatures elevated in the low to mid 50s. Sheltered areas that are
able to briefly decouple will be able to see lows dip into the upper 40s.

Short term / 6 am Saturday morning through Monday. During this period, potent northern stream energy
will dive southeastward from the Pacific northwest, noted by a strong PV anomaly dipping as far
south as the four corners region. This PV anomaly will help to dig a deep longwave trough across the
central Contiguous U.S., but much of the corresponding northern stream jet dynamics look to be
displaced further north as the initial PV surge becomes pinched off from the larger scale flow.

Meanwhile, a surge in the southern stream subtropical jet looks to round the base of the longwave
trough, with the 12z guidance now coming into better agreement with a cutoff low developing across
the lower plains out of the remnant northern stream PV lobe, while the northern stream jet remains
displaced to the north, and a split-flow regime setting up for the weekend and into early Monday.

Initially in this split flow, it looks to be one last dry day on Saturday with gradually increasing
cloud cover and temperatures once again reaching well into the 70s with lows Saturday night in the
50s. Then for the latter half of the weekend, a cold front will cross the region late Sunday into
Sunday night, with the trend in the past few runs continuing to indicate a slowing in the timing of
the front. This front will bring scattered to numerous showers riding a surge of deeper moisture,
with rainfall amounts generally on the order of one quarter to three quarters of an inch of rain.
Temperatures on Sunday look to spike again into the 70s despite thickening cloud cover and
increasing chances for rain from west to east late in the day.

Long term / Monday night through Thursday. While there continues to remain uncertainty in the
forecast, there are increasing trends that a strong low pressure system ( most long range guidance
deepens it to sub-990 hpa by the midweek period! ) will develop in the vicinity of southeast
Michigan for early next week. This low pressure system looks to develop in response to another surge
in northern stream PV energy diving south out of the Northern Plains, and interacting with the
ejecting upper- level closed low in an active southern stream. Because of the split- flow regime
leading up to this event ( long range guidance tends to struggle handling this regime beyond 3 days
), the degree to which phasing and amplification of the synoptic pattern occurs is still in
question, and should hopefully be resolved in a clearer picture in the next 24-36 hours.

Will go with a blended approach of the 12z GFS/ECMWF and various ensemble members during this
timeframe, with the forecast depicting likely showers Monday night through Tuesday night. Monday
night into Tuesday, there are signals for a potential heavy rain/high wind event ultimately
depending on the evolution of the low pressure system, and this trend will need to be monitored
closely over the coming days. A drying trend in the wake of the system will be possible to end the
long term period, but again will hinge on the evolution of the low pressure system.

Temperature-wise for the long term period, a trend back towards normal will occur, as much colder
air is ushered in within the strong northern stream flow. Daytime highs will return to the upper 40s
to upper 50s, with lows in the upper 30s to mid 40s.

. . posted at 357 PM EDT Fri Oct 20 2017