Thursday, October 20, 2011

Wild and wet day Chicago

Chicago (and much of the upper midwest) had quite the rainy and windy weather starting yesterday and into today.  So what's causing all this?

First, let's check the upper-level pattern.  Below is last night's 300mb analysis.  You can see that there is a deep, cut-off trough over the Mississippi River valley.  Strong jet streaks are notable on either side of the trough axis.

Most notably, the exit region of the eastern jet streak--where air is leaving the jet--is almost right over the location of the surface low.  Since the exit regions of jet streaks (particularly cyclonically-curved jet streaks) tend to be associated with areas of divergence aloft, this helps lower the pressure at the surface.  Notice in the surface map below that the surface low pressure center was right over Ohio--right under that exit region.
That's a rather spectacular surface low, there--the lowest pressures are down under 988mb.  Notice also that the isobars--the pressure contours--are very closely packed together, particularly on the northwestern side of the low.  Closely-packed isobars imply a very strong pressure gradient, and strong pressure gradients lead to high winds.  Very high winds.

As of 11AM this morning, here's the highest wind gusts as reported by WFO Chicago:

SITE                      PEAK GUST

WAUKEGAN HARBOR..............59
CHICAGO CRIB.................62
CALUMET HARBOR...............52
BURNS HARBOR.................55
MICHIGAN CITY................62

So over 60mph wind gusts in some locations.  There was also a 73 mph wind gust reported up north in Racine, Wisconsin.  It looks like sustained winds have been measured in the 20-30 mph range for much of that area.  Wave heights on Lake Michigan are also reaching upwards of 15 feet.  This is helped by the fact that the winds are generally blowing out of the north, meaning that they travel a long ways down the lake before reaching the shore.  This gives waves pushed by the wind a long distance over which they can build up height.  You can see a good video of the wind and waves from the WGN news site here.

Another story has been the rainfall with this storm.  Since the upper-level low has been cut-off from the mean flow, it has been slow to move out of the region.  As such, this storm and its associated rainfall have lingered over the area for over 48 hours.  Here's a map of the precipitation totals from yesterday and today:

Some areas in northern Indiana and Ohio have seen upwards of 3 inches over the past 24 hours.  Also notice in northern Michigan how there's an area of higher precipitation totals on the western shore of Lake Huron. With the low pressure center over Ohio and counter-clockwise winds around the low, you can imagine that in northern Michigan the winds have been out of the east.  Therefore it looks like we're seeing some lake effect enhancement of the rainfall as those easterly winds blow from the lake into northern Michigan.  I'd also suspect that the band of higher precipitation seen in northwestern Indiana is also due to lake effect enhancement as those winds come out of the north down Lake Michigan.

Winds today are still pretty strong, but they've backed to become northwesterly across the Chicago area.  Here's a surface map from the College of DuPage site valid at 18Z today.
You can see that winds speeds are still at 15-20 knots, even inland, but the winds have become more northwesterly.  Also note that the temperatures are really starting to cool off--it's only in the mid 40s everywhere.

This storm is forecast to finally move off to the east in the next 12 hours or so, leaving behind much cooler temperatures.  There are frost and freeze advisories in effect for much of the upper midwest as cold air moves in behind this powerful storm.

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