Hot/humid start to the week yields to a refreshing finish
Early this week we will be on the northern edge of a fairly flat heat ridge centered over the southeastern US.
However, this is all about to change as a strong upper ridge building over the eastern Pacific will help to amplify the pattern downstream over North America later this week.
We begin with the hot and humid weather to start. Highs on Monday will be in the low 90s. There will be a chance for showers and thunderstorms Monday night ahead of a shortwave.
With slightly warmer 850mb temperatures on Tuesday combined with downsloping, we will likely see readings in the upper 90s. In addition, dewpoints in the low 70s will make it feel like it is over 100.
Enough instability may be generated Tuesday afternoon for thunderstorms to develop near a prefrontal trough. With NAM predicted mlCAPE of over 1000 J/kg and 0-6km shear values of 20-30 kts, there would be the potential for some isolated severe storms, with SPC highlighting northern NJ on northeastward as being in the slight risk zone. A cold front will cross through Tuesday night.
There will be a sharp thermal gradient over the northeast on Wednesday behind the front.
The 10°C 850mb isotherm will be breaking into upstate NY. As such, some areas there will not make it out of the 70s. However, the cooler air will not be able to advect into the metropolis fast enough with highs in the big cities still in the upper 80s, though temperatures will be rather quick to drop off late in the day.
In response to the more amplified pattern upstream, a trough will roll through Wednesday night depositing some much cooler air over the region.
Modified Canadian high pressure will slowly build into the area through the end of the week.
High temperatures are expected to be in the low 80s on Thursday with low humidity. Temperatures in the urban areas will fall into the mid and upper 60s at night with 50s in the outlying areas. It will be a very refreshing finish to the work week. As to be expected, we will likely see a gradual warming trend towards the weekend with rising heights; however, a weakness remaining just off the coast, courtesy of strong blocking over the Atlantic, may keep us temporarily protected from the real surge of heat associated with a building ridge over the country’s mid-section.
Enjoy the week
Updated: 6:47 PM GMT on July 13, 2011
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Distinguished duo of troughs to dominate this week’s weather
Our current pattern consists of a persistent large upper low in the Gulf of Alaska, a strong upper high pressure ridge extending from the western US into the southeast, and opportunity for troughing to dig into the northeast. An upper low is currently moving east of Greenland and will be allowing for ridging to build once again near the island, reinforcing the negative NAO pattern. We can also see high pressure near the pole, which is indicative of a negative AO.
The center of an upper level trough situated just off the New England coast will remain closed off for a time as it has become partially cut off from the main flow. This has allowed for another day of dry and fair weather as a stable air mass remains in place. This low will be picked up on Sunday by a much stronger upper level low pressure system moving across Hudson Bay (see image above). In the meantime, weak shortwave ridging will get flattened out tonight as that same upper low moves into eastern Canada. As a result, we lose the subsidence and an approaching cold front will provide the needed lift for showers and thunderstorms to develop on Sunday in the now unstable environment. CAPE values increase to 500-1000 J/kg by late Sunday per the NAM.
However, shear will be significantly lacking, with roughly 10-20 kts of 0-6km shear expected.
For this reason, organized severe weather is not expected.
Independence Day will be mostly dry. The only potential fly in the ointment is a weak disturbance that is progged to move through. As a result of this, the NAM is printing scattered thunderstorms in the NYC metro area around the time of the Macy’s fireworks show.
Projected CAPEs are even a bit higher than Sunday in this area, but once again, shear values are very low so anything that could develop would be unlikely to become severe. The potential for pop-up storms is just something to keep an eye on, despite precipitation chances being very low.
The trough associated with the upper low over eastern Canada will swing through on Tuesday. This general pattern will continue to keep us shielded from the real heat and humidity. High temperatures are expected to remain in the upper 80s, averaging near to slightly above normal.
In the meantime, another large piece of energy will be ejecting out of the Gulf of Alaska trough, traveling around the ridge and then coming down the chute late in the week.
As such, another cold front will approach on Thursday with showers and thunderstorms expected, possibly lingering into Friday.
Canadian high pressure then builds in for next weekend with highs in the 80s along with comfortable humidity levels.
Looking ahead into the extended, I would argue for a continuation of temperatures averaging slightly above normal with little change in pattern. First of all, the 8-10 day average height projection on both the ECMWF and the GFS continue to show a trough in the Gulf of Alaska.
During the summer months, teleconnections downstream from a GOA trough support a trough in eastern Canada and the northeast.
Secondly, the NAO is expected to remain negative, which also supports a mean trough remaining over eastern Canada. Additionally, the AO is projected to remain negative, which would indicate maintenance of high latitude blocking with lower pressures underneath over Canada.
It is also interesting to note how both the observed AO and NAO have been riding the lower limit of the projected ensembles (note the 14 day forecasts for each of the indices above; black line is the observed index and red lines are the outer limits). This would argue for us to follow the lower range of the ensemble members, which would keep both the AO and NAO negative through the next two weeks.
While a negative NAO by itself wouldn’t guarantee freedom from heat waves, that feature combined with the GOA trough and a persistent weakness in the ridge over the eastern United States will likely be enough to shield us from oppressive heat during the first half of July.