May Weather Summary

Overview
May 2014 was the first month since last fall that did not result in well below normal temperatures across the region.  An active jet stream brought multiple rounds of active weather, along with significant temperature fluctuations.  The month started off with unusually cold weather, when temperatures ran around 10 degrees below normal on the 1st and 2nd.  Another cold snap occurred for several days during the middle of the month, when readings were 10 to 20 degrees below normal.  This even resulted in some late season patchy frost for parts of central and western Illinois on the morning of the 17th.  Besides these cold snaps, temperatures were above to well above normal, with most days seeing highs in the 80s and even a few 90s, which is more typical for mid-summer. 

Several episodes of strong to severe storms occurred during the month.  This was due to a typical spring weather pattern, where warm and humid air moving north from the Gulf of Mexico interacted with lingering cooler and drier air to the north.  Summaries of a few of the move significant events are below.  The convective nature of the rain events resulted in highly variable rain totals, but generally below to well normal totals for much of the area.

Saturday Night into Sunday Morning (May 10-11)
A warm front lifting northward toward central Illinois served as the focusing mechanism for showers and thunderstorms Saturday night into early Sunday morning.  An upper-level disturbance tracking eastward through the Plains interacted with the front to produce a cluster of storms across Missouri during the evening that tracked east-southeast along the boundary into south-central Illinois.  The strongest storms remained further southwest across Missouri: however, storms with locally heavy rainfall spread as far north as a Winchester to Pana line.  Several weather observers in this area reported rain totals of 1.50"-2.00".

Sunday afternoon and evening (May 11)
As the warm front lifted northward, a very warm and humid airmass arrived across central Illinois on Sunday.  With temperatures climbing well into the 80s and dewpoints reaching the upper 60s to around 70, the atmosphere became very unstable by afternoon.  Luckily there was no clear boundary to focus storms along: however, widely scattered thunderstorms popped up in the highly unstable environment.  One cell tracked northward through portions of Richland and Lawrence counties, producing penny-sized hail 3 miles west of Birds (Lawrence County) and golf ball-sized hail near Sumner (Lawrence County).  Another storm dropped quarter-sized hail in Shelbyville and destroyed a metal building and knocked trees down just east of Shelbyville.  Other storms produced tennis ball-sized hail near Greenup (Cumberland County) and baseball-sized hail in Towanda (McLean County)!  

Monday evening (May 12)

An approaching cold front triggered a broken line of strong thunderstorms from central Iowa southward into northern Arkansas Monday afternoon.  The storms tracked eastward across the Mississippi River into west-central Illinois during the evening.  Despite a very unstable airmass, weak low-level wind-shear and the presence of an anticyclonic circulation aloft prevented the storms from becoming severe.  40-50 mph wind gusts were common along the leading edge of the storms as they tracked eastward through the Illinois River Valley and across the I-55 corridor.  Some minor wind damage occurred in Tazewell County, including some tree limbs being blown down in East Peoria and a few trees and power lines being knocked down in Pekin.

Wednesday afternoon and evening (May 21)
Severe thunderstorms developed along a NW to SE oriented outflow boundary across east-central Illinois during the afternoon and evening of May 21st.  The storms were fed by an extremely unstable airmass characterized by Convective Available Potential Energy (CAPE) values of 3000 to 4000J/kg.  This highly unstable environment led to vigorous updrafts and the subsequent production of very large hail.  The largest stone reported was a whopping 4 inches in diameter (about the size of a grapefruit) on the north side of Tuscola!  Several other reports of hail larger than golf-balls were observed across portions of Douglas and Piatt counties as well.  In addition to the very large hail, the thunderstorms tended to train over the same locations for several hours, producing locally heavy rainfall and flash-flooding from far eastern McLean County southeastward across Champaign, Vermilion, northern Piatt, northern Douglas, and northern Edgar counties.  The highest observed rainfall totals were 3.84 in Broadlands (Champaign County) and 3.76 at Willard Airport in Savoy (Champaign County).

Temperature and Precipitation Maps 

 

 

 

  May Temp. Departure from normal

May Precip. Total

May Precip. % of normal 

 

Climate Data

The table below summarizes May 2014 precipitation, snowfall, and temperature, and departure from normal for selected cities across central and southeast Illinois.  Data from Peoria and Springfield are from ASOS sites, while others are from NWS Cooperative Observers.
 

Site Precipitation Departure from Normal Average Temp. Departure from Normal
Charleston

2.53" 

 -2.09"

 65.9

 +1.7
Decatur

4.04" 

 -0.75"

 63.8

  -0.4
Galesburg

2.52" 

 -1.93"

 61.9 

 +0.5 
Jacksonville

3.70" 

 -1.11" 

 64.0

 +1.9
Lincoln

1.78" 

 -2.38" 

 64.5

 +1.5

Normal

3.18" 

 -1.46"

 62.7

 +1.6
Olney

3.33" 

 -2.50"

 64.3

 +0.5
Peoria

1.45" 

 -2.88"

 65.8

 +3.4
Springfield

2.76" 

 -1.48"

 66.5

 +2.8
Urbana

4.51" 

 -0.38"

 63.2 

 +0.7


Links below are the monthly climate summaries for area cities. Only the summaries for Peoria, Springfield and Lincoln are considered "official", meaning they are the station of record for their respective locations. The other summaries are "supplemental", meaning another location in the area is the official climate station for that city.

Climate data for other area cities is available at http://www.nws.noaa.gov/climate/xmacis.php?wfo=ilx



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