The heat is on

With temperatures in the high 90s and the heat index well above 100°F, many are wondering how long this heatwave will last.

The most widespread heatwave in several years was in full swing across the central and eastern USA on Wednesday (July 20), with excessive heat warnings, watches and advisories covering portions of at least 30 states. High temperatures in Michigan during the past two days ranged from the upper 80s along lakeshore areas to the mid-90s in many other sections of the state. Combined with unusually high humidity readings – dewpoint temperatures have been running in the 70s during the same time frame – heat index values, or what the air temperature really “feels” like given the combination of heat and humidity, have soared above 100°F during the afternoon hours, stressing humans, livestock and crops.

The heatwave conditions are the result of a massive ridge of subtropical high pressure entrenched across much of the country since last week (Photo 1). With this pattern, the jet stream has generally retreated northward into Canada, leaving most of the lower 48 states under the influence of high pressure and fair weather. One notable exception has been the Pacific Northwest region, located beyond the periphery of the ridge, where weather has been persistently cool and wet. Given the recent upper air pattern, precipitation totals in most sections of Michigan have fallen below normal since late June, which is resulting in increasing levels of moisture stress for many crops, especially those on coarse-textured soils.

Heatwave conditions
Click on image to enlarge.
Photo 1. Air flow at approximately 20,000 feet above sea level, 8:00 EDT Monday July 18, 2011.
Winds are given at each of the grid points in terms of a wind arrow, which depicts both wind direction
(arrow points in the direction wind is coming from) and wind speed (add up the notches on each arrow:
the short notches are 5 knots (just above 5 miles per hour), long notches are 10 knots (about 11 miles
per hour), and black triangles are each 50 knots (about 55 miles per hour). Blue shading indicates
areas with winds in excess of 50 knots. Isotherms depicting temperatures at this level are given by
the dashed red lines (°C). Background figure courtesy of NOAA Storm Prediction Center, Norman, Okla.

So, how long will this pattern last? Current indications are that the main axis of the upper air ridge will very slowly edge eastward during the next few days, further intensifying heatwave conditions across the eastern USA. In Michigan, we will likely see the heatwave peak Wednesday (July 20) and Thursday (July 21), with maximum temperatures reaching the mid- and upper 90s in many areas of the state. Slightly cooler (although still well above normal) temperatures are likely this weekend. In terms of rainfall, we can expect a chance for scattered showers and thunderstorms on an almost daily basis through the weekend. Unfortunately, areal coverage with any rainfall will likely be limited, with highly variable rainfall totals. Some areas will remain dry, with ever-increasing levels of moisture-related stress.

The next best chance for more widespread rainfall will be late this weekend in association with a weak, cool front forecast to move through the state. That front should also bring temperatures back to more seasonable levels by early next week (upper 70s north to low and mid-80s south). Further ahead, current medium range forecast guidance is suggesting that after a short break early next week, there will be a reestablishment of the upper air ridging pattern across the central USA. NOAA Climate Prediction Center 6-10 and 8-14 day outlooks (covering July 25-29 and July 27-August 2) both call for a continuation of above normal temperatures. Precipitation totals are forecast to range from near normal levels across Lower Michigan to above normal levels across the Upper Peninsula. With this type of upper air pattern, my personal guess is that precipitation totals during both periods will remain in the normal or even below normal category.

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