NWS Area Forecast Discussion
Area Forecast Discussion National Weather Service Chicago/Romeoville Illinois 320 PM CST Monday january 28 2013 .Discussion, 257 PM CST Very mild and moist conditions will continue to advect north across northern Illinois/northwest in this evening, as the atmospheric river progged to stretch from the western gulf of Mexico north to the Ohio valley/Great Lakes regions establishes itself. Temperatures this afternoon have remained in the mid/upr 40s across the cwfa with a few areas south of I-80 warming into the low 50s. Dew points have steadily risen throughout the afternoon into the low/mid 40s. Visible imagery shows considerable cloud cover across the region, however a small break in the clouds early this afternoon did occur across northern Illinois stretching north into southeast Wisconsin and central Lake Michigan. The warm frontal boundary has already pushed north to central Iowa stretching east through central Illinois/Indiana. South of this line across central Missouri temperatures have warmed well into the 70s with dew points in the upper 50s, which is exceptionally rare for late jan. This record warmth will approach the cwfa overnight, and provide the expected record highs for Tuesday across the region. Overnight the focus remains on the increasing low level moisture and potent jet. 850mb winds will steadily increase from the south/southwest to around 30-40kts, then Tuesday will continue to increase. Directional shear will be steadily increasing throughout the night as elevated instability begins to indicate CAPE values around 200-400j/kg by 09z tue. Have maintained mention of convection overnight. The other challenge remains for this evening through the overnight, residing with potential for dense fog. A few obs this afternoon continue to indicate reduced vsbys around 1sm, with pnt hovering around 1/4sm. Given the addtl moisture lifting north advecting across slowly thawing and moist surface conds, it appears areas of dense fog will redevelop later this eve. The challenge is will be fall to dense criteria and for how long with the expected precip. So have opted to hold off on issuing a dense fog at this time, given the lower confidence on placement/timing. Confidence, medium. Tuesday, Tuesday appears to be the day that will produce record temperatures with guidance pushing highs into the 60s, and if skies were to thin slightly Tuesday afternoon a few areas along and south of a pontiac to Kankakee line could warm to psbly the mid/upr 60s. Temperatures will only be one parameter to be concerned with tue, as abundant moisture and impressive low level speed/directional shear are poised to push overhead. At this time spc continues to have the south/southeast portions of the cwfa progged for a slight risk of severe weather tue. Given such a dyNAMic and contrasting airmass between the warm/moist and cold, this appears to be such an anomalous setup that it wouldn/t be too much of a surprise for severe weather to occur. The largest concern is for damaging winds, although with the strong directional low level shear and a moist boundary in the vicinity of the cwforecast area, a few thunderstorm cores could see rotating updrafts. Which opens the door for an isolated tornado Tuesday afternoon. While the first half of Tuesday looks to be quiet, things will rapidly change for the afternoon/eve hours as the shortwave approaches from the west. The other challenge for tue/tue night will be on the potential for heavy rainfall, which could lead to flooding. Guidance continues to show some corrections with respect to the axis of highest precipitation totals, however it appears to be zeroing in on a central Illinois/Indiana stretching north through northeast Illinois/northern in. The combination of the atmospheric river, pwat values progged to approach 1.5" which ensembles suggest is 5 to 6 sigma above climo, have opted to proceed with a Flood Watch. Further complicating the flooding potential will be the still moist top few inches of soil. While it appears the top few inches of soil will thaw, it doesn/t appear the entire column will thaw. Which will further exacerbate the runoff or ponding concerns. At this time guidance points towards quantitative precipitation forecast totals of around 1.00" along and northwest of a lasalle to lake county Illinois line. East of this line forecast precipitation totals ramp up towards 2 inches. At this time the highest rainfall totals appear to be along and east of a Ford county Illinois to lake county in. Frontal passage has slowed slightly with the latest guidance, arriving across the northwest cwfa around 00z wed, approaching the central cwfa by 6-9z, and should eventually push through the far southeast cwfa by 12z Wednesday. Confidence, medium/high. Wednesday through Sunday, a return to winter is poised to slide back across the region Wednesday and continue through at least Friday, with a gradual moderation sat/sun. High amplitude anomalous 500mb ridge will steadily pivot east, as a deep trough slowly slides east across the Great Lakes Wednesday and lingers through the first half of the weekend. Further west a pac-nw ridge continues to be advertised to weaken throughout the next few days, which may aid in further de-amplification of the Great Lakes trough by sat/sun. Much colder air will continue to advect across the Great Lakes region, as a potent thermal trough of -18 to -20 degree c continues to push overhead thur. Guidance has indicated some weakening in the potency of the cold air. Much colder air will be displaced to the north across the upper Midwest/northern Great Lakes, where -24 to -26 degree c air at 850mb is progged. This is rather impressive given how anomalously warm Tuesday will be to transition in a 48hr period from warm/moist to arctic air thur. In the later periods confidence remains high that temperatures will slowly moderate back to seasonal, possibly leaning above seasonal conds by sun. Extended precipitation forecast outlook, after the potent shortwave progged to traverse the region cyclonic flow appears to linger through Thursday night. Then for fri/sat anti-cyclonic flow will begin to push the prolonged wrap-around moisture to the northeast of the cwfa. There appears to be a weak vorticity maximum that may rotate around the 500mb vort, and slide across the upper Midwest towards the cwfa wed/thur. Guidance suggests that enough lift could be around in this timeframe to push parcels into the dgz, warranting some light accums Wednesday to early thur. Then dry air pushes in for the weekend with the aforementioned temperatures returning to seasonal conds. Confidence, medium/high. Beachler && |
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