Highlight: MJO, heavy rains and hot weather ahead.
Fig. 1-3: Upper air pattern for days 1-5, days 6-10 and days 11-15 identify the influence of a pulse of MJO energy on the pattern.
Discussion: A cool upper trough across the East will depart this weekend (Fig. 1). During this process southern Canada turns warmer to the south of the polar vortex. ENSO is now neutral therefore the Madden Julian oscillation (MJO) becomes more active. MJO has already been involved with producing early season tropical cyclones in the West Pacific and East Pacific Ocean basins. The MJO forecast indicates an east-shift across the tropical West Pacific over the next 1-2 weeks. A streak of that energy reaches into the Southwest U.S. in the 6-10-day period to produce a low latitude upper trough which soaks the Mid-south U.S. with excessive rainfall, flash flooding and severe weather (Fig. 2). Once the MJO-induced rainfall rinses-out over Texas and vicinity the tropical nature of that event is released northeastward into the East States amplifying an upper ridge pattern (Fig. 3). In 10+ days 90F spreads into the Mid-Atlantic States with mid 90’s possible in the Southeast.
The GFS is quite dramatic with the evolution and east transition of the MJO (Fig. 4). Other models are less impressed with MJO’s involvement with the forecast pattern. If the MJO does not strengthen and shift east the excessive rainfall and following heat risk indicated by the GFS at midday will be weaker.
Also note the returning upper trough in the Northwest U.S. in the 11-15-day period. This feature is characteristic of negative Pacific decadal oscillation (-PDO) when waters are cooler than normal off the West Coast. The PDO index for April (Fig. 5) remained moderately cool at -0.91. -PDO is normally present with La Nina. Note the lingering borderline La Nina index character of the Nino34 SSTA observations.
The specific potential rainfall ahead of the Southwest U.S. trough centered on the 6-10-day period is impressive! GFS indicates 2-5 in. of rain across the southern Great Plains to Texas (Fig. 6). Severe weather will strike Texas! Meanwhile, the GFS ENS threshold forecast indicates widespread 90’s in the 11-15-day period including the Mid-Atlantic States (Fig. 7). The warmth is triggered by the heat release of the MJO-induced rainfall.
Fig. 4: The active and intensifying 14-day MJO forecast by the GFS (top).
Fig. 5: The PDO index and Nino34 SSTA index for the past 12 months.
Fig. 6: The GFS ENS day 11-15 >90F risk.
Fig. 7: The GFS 10-day rainfall forecast issued at midday for Texas and vicinity.