Prognostic Discussion for Week 3-4 Temperature and Experimental Precipitation Outlooks
NWS Climate Prediction Center College Park MD
300PM EST Fri Mar 15 2019
Week 3-4 Forecast Discussion Valid Sat Mar 30 2019-Fri Apr 12 2019
During the past month El Nino conditions have become further entrenched in the tropical Pacific, with an oceanic Kelvin wave helping to reinforce the available pool of warm water near the Date Line, while also serving to amplify convective and zonal wind anomalies. This has helped to stablize the Pacific-North American (PNA) pattern into its positive phase, with models continuing this signal through the next few weeks. The enhanced phase of the Madden-Julian Oscillation (MJO) is presently across the Maritime Continent, with dynamical model guidance suggesting its imminent demise. These outlooks are to be taken with caution, as the long-term mean (that is removed from these outlooks) is biased in Phases 6/7 tied to the amplifying El Nino conditions and fairly active equatorial Rossby wave activity in the West Pacific since 2019 began. Further complicating things, some of the models may be latching onto an atmospheric Kelvin wave over the Western Hemisphere, that is acting to weaken the apparent MJO in some observational constructs, but in reality is just splitting the difference between two centers of action. Given this scenario, coupled with documented difficulties of dynamical model guidance in propagating the MJO across the Maritime Continent, the active envelope making it to the Pacific over the next week or two appears to be a reasonable assumption. An additional consideration is the potential for a final stratospheric warming being hinted by dynamical model guidance, although purported impacts from the stratosphere on North America are typically highly inconsistent, while dynamical models have well-known difficulties with stratospheric-tropospheric coupling. The current Weeks 3 and 4 outlook utilizes a blend of statistical guidance (leveraging the MJO, El Nino, and decadal trends) with our typical dynamical model suite of the CFS, ECMWF, and JMA. Subseasonal experiment (SubX) guidance was also consulted, while the anticipated evolution from the Week-2 pattern is an additional consideration.
Model guidance consistently features a PNA+ flavor and amplified wavetrain emanating from the Tropical Pacific across the higher latitudes of the Western Hemisphere. This leads to the most pronounced fields across the CONUS being anomalous ridging over the Northeast and more modest anomalous troughing across the Four Corners. Positive height anomalies are negatively-tilted from Northern Alaska through the Panhandle, with anomalous troughing located south of the Aleutians. The broad consistency among dynamical model guidance closely mirrors the anticipated lagged El Nino and MJO circulation responses this week, helping to increase confidence in the overall forecast. The only major outlier among guidance is the constructed analog conditioned on 200-hPa streamfunction, which features an anomalous trough across the Canadian Rockies, a sharp departure from the Week-2 model forecasts of a blocking high situated here.
The resulting Weeks 3-4 temperature outlook tilts warmer than normal across much of the country with the exception of parts of the southwestern CONUS and the High Plains. The highest confidence for above-normal temperatures exists across the Northeast, closest to the forecast position of the anomalous ridge axis. Probabilities for above-normal temperatures decrease while shifting southward and westward, although there is an extension of above-normal probabilities through the Pacific Northwest tied to the anticipated anomalous ridge across the Canadian Rockies. With the troughing forecast to undercut this ridge, below-normal temperatures are favored across Southern California into the Four Corners region, with the best chances for cooler than usual temperatures focused near Las Vegas. Equal chances are forecast from Northern California through the Central Plains and southward across much of the High Plains, given uncertainty regarding how progressive the trough is. The entirety of Alaska also tilts towards warmer than normal temperature chances linked to the forecast ridge over the eastern part of the state, with some reduction in above-normal temperature probabilities across the North Slope.
Precipitation chances skew towards below-median probabilities across the northern tier of the country, tied to above-normal height expectations. The mean storm track would appear to be for shortwaves to cross through the Four Corners and potentially undergo lee cyclogenesis across the Central Plains before a track toward the Midwest. Above-median precipitation odds are increased from the Central Great Baisn through Corn Belt regions, with the highest probabilities for the Central Rockies. Depending on how progressive the trough in the West becomes, it could tap into moisture from the Gulf of Mexico, resulting in uncertainty for the southeastern quarter of the CONUS and equal chances. Model guidance consistently supports below-normal precipitation across Northern Mainland Alaska and the Alaska Panhandle tied to the ridge axis presence, while above-median precipitation odds are increased upstream over the Aleutians and westernmost parts of the Alaska Peninsula.
Observed SSTs around Hawaii range from -0.25 to -1.25 degrees C. This scenario favors a slight tilt towards below-normal temperatures being favored in the present outlook, with the exception of Hilo where SST anomalies are the smallest. The cool SSTs and low frequency state in the tropical Pacific both favor a stronger tilt toward below-median precipitation across the entire island chain.
| Temperature | Precipitation |
| FCST | FCST |
| Hilo | EC | B60 |
| Kahului | B55 | B60 |
| Honolulu | B55 | B60 |
| Lihue | B55 | B60 |
Forecaster: Daniel Harnos
The next week 3-4 outlook will be issued on Friday, Mar 22, 2019
These outlooks are based on departures from the 1981-2010 base period
These are two category outlooks and differ from official current three category outlooks currently used for the monthly and seasonal forecasts.
The shading on the temperature map depicts the most favored category, either above-normal (A) or below-normal (B) with the solid lines giving the probability ( >50%) of this more likely category (above or below).
The shading on the precipitation map depicts the most favored category, either above-median (A) or below-median (B) with the solid lines giving the probability ( >50%) of this more likely category (above or below).
In areas where the likelihoods of 2-week mean temperatures and accumulated precipitation amounts are similar to climatological probabilities, equal chances (EC) is indicated.
As of May 19, 2017, the temperature outlook is operational, while the precipitation outlook is still experimental