July 17, 2011: Severe Storms Tomorrow

Note: This blog is going on a weather related trip over the next week. The next update, along with some comments on the trip, will be posted on Saturday, July 23.

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A massive ridge is currently centered over the central United States, bringing extreme heat to that region. Some of the heat from this ridge is spreading towards the region, which is bringing temperatures into the 90s. Meanwhile, a storm in Canada will bring a cold front through tomorrow, bringing a risk of severe weather. Behind this front, temperatures will slightly cool down, but by late next week, temperatures will warm up even more, potentially reaching the 100 degree mark in parts of the area.

Today’s Outlook:

The outlook for today has not changed since yesterday. Mostly to partly sunny skies are still expected, with highs in the upper 80s to lower 90s from NYC and further west, and the mid 80s across most of Long Island and southern Connecticut.

Monday: Severe Storms Possible

A cold front will move through the region on Monday, bringing a risk of strong to severe thunderstorms. Unlike most of the outbreaks we’ve seen in the region this summer, however, this set up is more favorable for severe weather. In addition to supportive lift index and CAPE values, bulk shear is also modeled to be supportive, between 35 and 45 knots. We had some cases where there was favorable bulk shear this summer which did not end up over the areas which saw severe weather, but in this case, the supportive area of bulk shear is expected to end up over the same region expected to see thunderstorms, which is already a sign of a better chance of severe thunderstorms. Lapse rates are expected to be supportive as well.

With the factors above in place, there is at least a 15% risk of severe weather across most of the area. The best risk of severe thunderstorms is expected to end up in southern New England and parts of southern New York State, which includes the northern parts of the area. These places have the most supportive parameters, and at this time, I placed a low-end 30% risk in those areas. Thunderstorms across the area will be capable of producing gusty winds, heavy rainfall, and small hail.

Medium Range Heat Wave: 100s Possible

Behind this cold front, temperatures will only slightly cool down. Tomorrow will bring lower to mid 90s from NYC and further west, and Tuesday will bring upper 80s to lower 90s from NYC and further west. Mid to upper 80s are expected as well in Long Island and southern Connecticut. Wednesday will bring similar temperatures to those of Tuesday.

Things will change by Thursday, however. The large ridge currently over the central US will shift eastwards, so that the focus of the heat will be further east as well, closer to the area than we’ve seen most of this summer. A strong cold air mass in SE Canada will attempt to drop southeast towards the region, but at least early in this time frame, will only make it as far south as northern New England, allowing the excessive heat to spread into the NYC area.

Forecast for the Tri-State Area: Temperatures on Thursday will begin to warm up, reaching the lower to potentially mid 90s inland, mid 90s across most of the immediate NYC area, and with a southwest wind, Long Island and southern Connecticut should reach the mid 80s to lower 90s. Humidity will begin to slightly increase as well, allowing the heat index to reach the upper 90s in the immediate NYC area.

Friday is likely to be the hottest day of this heat wave, though Saturday could bring similar conditions as well. 850 mb temperatures are modeled to be in the 21-24c range, similar to what we saw with the July 2010 heat wave. Mainly dry conditions are expected along with mostly/partly sunny skies and a WSW wind. Unless there is a significant change with the expected set up, the conditions above are capable of bringing the hottest day of the year across the area. High temperatures are likely to reach the mid to upper 90s inland, upper 90s to potentially lower 100s across most of the immediate NYC area, and Long Island/southern Connecticut are likely to end up in the upper 80s to mid 90s, though upper 90s could be possible closer to NYC in the warmest case scenario. Humidity is expected as well, with dew points modeled to be near the upper 60s to lower 70s. This would bring the heat index into the mid-upper 100s in the immediate NYC area.


Forecast Uncertainty: The forecast for Friday is not 100% certain yet, but based on the set up for this time frame and the agreement between the models and model ensembles regarding the intensity and placement of the heat, there is a relatively high probability that Friday ends up bringing conditions similar to those mentioned above. There is the possibility that the heat ends up being slightly weaker than expected or that the back door cold front drops south of the expectation, which would result in cooler temperatures than the above, but even in these scenarios, Friday is likely to end up as a hot and humid day.

Beyond Friday, there is increasing uncertainty regarding another cold front, as the GFS has the heat lasting through early next week, the ECMWF brings a weak back door cold front through which slightly cools down temperatures, and the GGEM brings a stronger cold front through with temperatures returning into the 80s. At this time, I am siding with a scenario closer to that of the ECMWF, where temperatures cool down into at least the upper 80s to mid 90s on Saturday or Sunday, followed by another warm up by early next week. There is more uncertainty, however, and the confidence level in the forecast decreases beyond Saturday. With the next update, either late on Friday or on Saturday morning, I will discuss what the forecast is from that point and beyond.

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